Speaking of the proverbial c*nt-hair, a machinist/millwright friend of mine once tired of the expression and actually did something about it. Obtaining specimens of same from 3 brunettes, 2 blondes and a single redhead, he measured them with his nice Starrett mike, added and averaged and proclaimed the precise calibration of the WWSCH: the Welbanks World Standard C*nt Hair.
It came out at precisely .0035".
After that, any time somebody said "tighten her up just a c*nt-hair" or "mill it down just a c*nt-hair", he would work to a very precise three-and-a-half thou.... and he was generally very, very close to precisely what was desired!
BTW, the word "c*nt" is not a vulgarism: it is the correct name for that part, in Anglo-Saxon English. The "polite" word used currently is Latin, meaning "sheath" and was brought into the language in the 18th Century by some frustrated classicist who thought that anything 'common' was unfit for a 'gentleman' to utter. They "cleaned" the whole dictionary at that time, rewrote it, leaving out all the "impolite" words..... which were actual ENGLISH..... and substituting Latin words (generally meaning exactly the same, or else euphemisms which can get downright funny at times) that the common clay didn't understand.
Sound silly? Only until you remember that the Imperial Russian upper class spoke French because Russian was too common, and the upper-class Romans spoke Greek because ordinary Latin was 'vulgar'..... which actually means 'common'.
Oh, it's fun!