When, and how did the CH become the standard unit of measure in the construction industry?(possible NSFW).

Twice today, and today is not an unusual day, I heard the term “Cunt Hair” (as in “It’s too long, take of a cunt hair” and “Move it over a CH.”) on the jobsite. Two different trades, two different times.
So, how did this become the standard that all things are measured by at least in construction? Why did it? Is it used in other lines of work? How about scientific research? How far back does its’ usage go?
What did they use as the standard unit during the Dark Ages when lice made shaving a necessity and merkins were all the rage?
I heard that there may be an Egyptian Hieroglyph for it so that could put it at over 4000 thousand year old and there are many undecifered cave paintings some of which could easily refer to this!
These are the types of questions that keep me up at night.

Help!