Foolproof vs. full-proof

The adjective foolproof means infallible or, more literally, impervious to the incompetence of fools. Just as a bulletproof vest makes one invulnerable to bullets, a foolproof plan is designed to be invulnerable to fools. Foolproof is usually one word, without a hyphen (though the hyphenated form, fool-proof, is not uncommon).

The word is occasionally misspelled full-proof. There are arguments to be made in favor of this spelling (see the comments below for a couple of them), and of course anyone who likes it is free to use it, but it is not the conventional spelling (i.e., the more common one and the one listed in dictionaries) and is likely to be seen as a misspelling by some readers. 



The public knows that no security system can be foolproof, but some acknowledgement of accountability is warranted nonetheless. [Milwaukee Journal-Sentinel]

This is not foolproof from a security standpoint since the malicious individual could simply configure their computer with an IP address on the network. [Digital Evidence and Computer Crime, Eoghan Casey]

Apparently, DNA testing was seen as so foolproof that alibis didn’t count. [Herald Sun]

It can be tweaked according to the talent at hand, but nothing is foolproof, especially with young players. [Sports Illustrated]

One foolproof method of contraception is sterilization. [Life: The Science of Biology]

Here, he shows a simple roasting technique that’s both foolproof and infinitely adaptable. [Globe and Mail]


Check Your Text


  1. unassuming says:

    I don’t use it but full-proof could make logical sense to someone who has had higher level of mathematic education.  Where a proof is the evidence of a theory so a full-proof could be a theory/plan that has been fully proven.

    • That would be a full (adj.) proof (n.). Using full-proof as a mathematical adjective in this contrived scenario would leave it with nothing to modify unless you were talking of a full-proof proof. That would be the mathematical equivalent of making a withdrawal from the ATM machine.

    • Or to anyone developing other kinds of “proofs” (such as scientist or a developer working on a “proof of concept”).

  2. David Johnson says:

    Wait now. When you proof something, you check it for errors. 

    The verb defintion:
    “to test; examine for flaws, errors, etc.; check against astandard or standards.”
    So proceeding from that, something full-proof has been fully tested and examined for errors, and there are none:

    It’s not correct, but it’s incorrect in a way that does make sense. 

    • Grammarist says:

      Good point. We might have to revise out that “makes no logical sense” bit and add a paragraph about the ways in which “full-proof” can sort of make sense. 

    • Wait now. “To proof” is short for “to proofread”, which is a corruption of “to read proofs”. (Cf. “to bartend”.) Proofs are test-printings, made /to prove/ or /to test/ the correctness of type-setting (or to test what a photograph will look like). That being so, “to proof” is not a proper verb. Even if it were, the derived adjectival form would be “proofed”, not “proof”, and the superlative would be “fully-proofed”, not “full-proof”. “Full-proof” neither is correct nor makes logical sense. It’s just a solecism for /foolproof/.

  3. RandomGuy says:

    or it could be used to indicate a high content of alcohol. something that is full-proof would be of the highest alcohol content. Applying to another situation could mean that it’s extremely strong or potent perhaps? “This new energy drink is full-proof’

    • sethshead says:

      200 proof energy drink? It might resemble a scene from “Raiders of the Lost Ark,” but I’d try it.

    • AnotherRandomGuy says:

      Full proof (100% proof) liquor is roughly 50% alcohol by volume. Hence the existence of overproof spirits like Bacardi 151, higher than full proof; one of the few cases in which a percentage higher than 100 makes some kind of sense

      • Miiiiiiiike says:

        That’s not how proof works. A proof isn’t a percentage. You wouldn’t have 100% proof, it would just be 100 proof or 50%. 151 isn’t over 100%, it’s over 100 proof but “only” 75.5%.

  4. sethshead says:

    I’m beginning to take a shine to “full-proof,” even though it’s a neologistic hypercorrection of “foolproof,” because it actually seems to fit current usage more accurately.

    Bear with me.

    “Foolproof” originally meant a product or plan that would still function as intended even if improperly used or ineptly followed. In the same sense as the more recent “idiot proof,” it implies some degree of defensive engineering. Today, however, as your first example attests, the meaning has evolved to something more akin to “fail-safe,” whereby system redundancies or astute planning eliminate the possibility of mechanical, rather than operator, failure.

    In that case, “full-proof,” which appears – appropriately – to have caught on first in the tech and engineering community, would make perfect sense, as something, like code, which has been checked for any possible design or process flaws, and found to contain none. It has been ‘proofed in full,’ thus “full-proof.”

    • Seldom Needy says:

      Also in a science/engineering discipline here. I find I rarely hear “fool-proof” anymore as it seems to imply that “idiots can use it without causing chaos.” While sometimes the case, I’ve seen “full-proof” used to mean “it cannot fail,” there under the assumption it will however be in *competent* hands.

      As such, I had always interpreted “full-proof” (many people actually do pronounce it this way) as sortof a way of saying it’s failure-proof/bullet-proof etc. Meaning the “full” is a shorter standin for “everything,” meaning – as you said – that it has the vast majority of contingencies already worked into its structure, even though the perfect airhead could still blow it up.

      • You are both just making excuses for the use of an incorrect word. Sure, if enough people get it wrong it eventually becomes right, but wrong is wrong, at least for now.

        I love the irony here, foolproof is not a word safe from fools.

        Edit – Wikipedia’s “idiot proof” page makes no mention of the misspelled version of the word.

        • Seldom Needy says:

          >People are idiots for using phrases that have mutated over time
          Yeah okay
          >Wikipedia is the final authority on how people should use words
          Elevated kek

          • If you’re educated enough to know the term is foolproof, you have no need to use the incorrect spelling. Only idiots (too strong a word really) got it wrong in the first place and subsequent idiots who didn’t bother to learn the correct word propagated the bad habit. The more idiots misuse something, the more likely it is to become acceptable. This will only get worse in the future as kids fail to learn how to spell words properly and just rely on the word processor/browser/phone auto-correcting or suggesting the correct spelling.

            The Wikipedia bit was an edit, I never went there to formulate my initial opinion. Perhaps the Urban Dictionary entries are more accurate :-p

            Either way, you’re just making excuses for poor spelling.

          • No, dude, it has a different meaning. — As does half-proof. They are similar but unrelated terms.

          • Categorically no.

            It would appear that half-proof is indeed a word (thanks Wikipedia) and is related to the term full proof (notice the space) in that we are looking at how much proof is needed in a court of law.

            But that is a completely different subject to what is being discussed; the point here is that people mistakenly use the term full-proof when they really mean foolproof.

            In England, generally people will pronounce fool more like full, so my guess is that it was simply misheard and misunderstood originally.

            I am from an area of England that doesn’t pronounce the oo sound as ‘uh’ (when sandwiched between other letters), and instead pronounces it ‘ooh’, so for me the difference between f ooh lproof is very clear compared to f uh l-proof.

          • John Arce says:

            Although I agree with (what I perceive) to be your stand on ignorantly continuing to misuse words so much that they become the norm, I feel your view from your high horse blinds you to a simpler truth. ALL words are made up by these “idiots” as you call them; humans. (Though I do feel most of them ARE idiots, this is besides the point)

            Plenty of words (I would say all, but I’m not in any way sure of that) are derived from other languages, older dialects, misheard or not. Just because you or I were born in a time when these words were/are the norm, doesn’t mean they are the first incarnation. Which should we then use? Why not the very first?
            Save for dead languages, every language most likely evolves, slowly over time. As the users of these languages, WE decide what is right, not some language-entity.

            Herein lies a point of flawed reasoning on your part, that I point out, to inform you of it, not to ridicule.

            You blame Wikipedia for coming up with the word half-proof. Wikipedia itself isn’t a conscious entity. It’s a repository for “knowledge” (for lack of a better term) This knowledge is entered by us, AFTER already being in existence. Theoretically, it should only end up there when it IS a word.

            Grammar rules were invented AFTER the fact, that’s partly why there are
            so many exceptions to those rules. They’re nothing more than a record
            of how we WERE doing it. The rules weren’t the source of language.

            As I said, I agree with the point that we shouldn’t let ill-informed “idiots” dictate what is right or wrong, but in the end you can’t stop the flow of change and in trying YOU (we) might end up being the “idiot”.
            This isn’t “an excuse for poor spelling”, just a fact of life, sad as it may be to some.

            I personally always took the word to BE full-proof, Not being a native English-speaker, didn’t automatically make me an idiot. Probably to my mind it made more sense for something to be full (100%) proof.
            “Only idiots got it wrong in the first place”; I don’t label myself, or anyone, an idiot for making a mistake, especially not in the first time I’ve encountered something.
            I’ll reserve that term for the last person to hold out that something is wrong or right, hopefully it won’t end up being you.

        • buenabitca says:

          The term “full proof” can be seen at least as early as 1810 in James
          Tilly Matthews’ ~Illustrations of Madness~, so this really needs to be
          checked in the OED. It’s premature (oh, my; a polite term in an Internet comment! ;) to accuse anyone of “idiocy.”

  5. Along the lines of the alcohol concept, which could easily apply to engineering/mathematical applications as well. Full means at capacity, 100%. Also, complete especially in detail, number, or duration. Being at the highest or greatest degree. However, used with “proof”, it would properly appear in a sentence as an adverbial phrase “fully proofed”. Reasonably fullproof or full-proof could work. Apple’s Auto-Correct accepts it both ways.

  6. uglyoldbob says:

    Wouldn’t something that is full-proof be incapable of being full?

    • Mohammad says:

      That’s an awesome point. So the only correct usage of full-proof would resemble a bucket with a hole in it. I smell a t-shirt idea…

  7. In Medieval criminal law, prosecutors saw half-proof evidence as stronger than full-proof evidence. See more about full-proof in Wikipedia and an article by Jill Lepore in this week’s New Yorker.

  8. Lets play a game kids, find the word that doesn’t belong
    bulletproof foolproof fireproof full-proof.

  9. gradyphilpott says:

    Full-proof will become the mandatory term when the fool’s anti-defamation movement gathers the requisite “steam.”

    It’s like using their as a singular pronoun, which was unthinkable back before women’s liberation, when most people had some common sense.

    • Hanii Puppy says:

      Their as an alternative to his/her, and they as an alternative to s/he, has been a thing far longer than using s/he has. It’s useful where gender isn’t clear or is intentionally being kept ambiguous, and isn’t as awkward to say in casual conversation as “he or she”, or “his or her”.

    • Linking loss of common sense with women’s lib is churlish and has no place in a grammatical discussion. If I were to invite you to save it for your fellow bigots, you might feel the same indignant rage your comment would engender in a supporter of gender equality.

  10. Don’t try to make excuses to continue using full-proof. The term is foolproof. It’s that simple. Stop using full-proof.

  11. Brandon4foot7 says:

    I frankly prefer full-proof just because I see it as a more literal translation of infallible. Also, I write articles on the internet for a living and don’t want to risk my readers thinking that I’m somehow calling them fools. I’ll take being diplomatic over being right any day of the week. It makes for less aggressive comments.

  12. kumpy pie says:

    nah this is stupid its def. fool proof

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