Sharp as a tack is an idiom that is decades old. An idiom is a word, group of words or phrase that has a figurative meaning that is not easily deduced from its literal definition. Often using descriptive imagery, common idioms are words and phrases used in the English language in order to convey a concise idea, and are often spoken or are considered informal or conversational. English idioms can illustrate emotion more quickly than a phrase that has a literal meaning, even when the etymology or origin of the idiomatic expression is lost. An idiom is a metaphorical figure of speech, and it is understood that it is not a use of literal language. Figures of speech have definitions and connotations that go beyond the literal meaning of the words. Mastery of the turn of phrase of an idiom or other parts of speech is essential for the English learner. Many English as a Second Language students do not understand idiomatic expressions such as in a blue moon, spill the beans, let the cat out of the bag, chin up, on the ball, barking up the wrong tree, kick the bucket, hit the nail on the head, under the weather, piece of cake, when pigs fly, and raining cats and dogs, as they attempt to translate them word for word, which yields only the literal meaning. In addition to learning vocabulary and grammar, one must understand the phrasing of the figurative language of idiomatic phrases in order to know English like a native speaker. We will examine the meaning of the idiom sharp as a tack, from where this expression is derived, and some examples of its use in sentences.
Sharp as a tack means mentally acute, clever, intelligent, mentally alert. The tack that is referred to in this idiom is a short needle or nail used in upholstery, shoe making, or affixing items to a wall or a bulletin board. Sharp as a tack is an example of an idiom that is a simile, which is a phrase used in a sentence that is a comparison of one thing with something else using the word like or the word as. This idiom involves wordplay with the word sharp–which may mean having a piercing point or may mean being mentally acute. The phrase sharp as a tack is most often used to describe an older person who is surprisingly mentally alert. The idiom came into use shortly after the turn of the twentieth century, replacing an older idiom, sharp as a needle.
While ALS has rendered Lobo unable to move from the neck down, Eric Lobo said his dad can still speak, and remains mentally “sharp as a tack.” (The Chicago Tribune)
Now they are both in their 70s; my mom is physically well but has dementia and my dad is still as sharp as a tack but has many physical problems that leave him frail and perpetually exhausted. (The Capital Gazette)
But once he understood the question he demonstrated that he is still as sharp as a tack. (The Fairfield Daily Republic)
There are certainly octogenarians who are physically fit, sharp as a tack, and as competent at work as any whippersnapper. (New York Magazine)