Having grown up in an army family, acronyms and military jargon (like AWOL — absent without leave) were a daily part of life.
One of my first encounters with Japan's 兵語 (heigo, military terminology) came about when Kato-san, an older co-worker, jokingly made a reference to the term M検 (emu-ken, an "M inspection"), which in the old days doctors performed at military induction physicals to look for visible symptoms of sexually transmitted disease.
Ken, I supposed, was short for 検査 (kensa, test or examination). But what did the M stand for? Then Kato-san drew a letter M on a sheet of paper, and I immediately understood it to be a crude depiction of the anatomical shape of the, er, object being examined.