Sharp as a tack is an idiom that is decades old. 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)