Enculer les mouches

Literally: To sodomize flies

Example: “On ne va pas enculer les mouches pour deux centimes de différence.” — We're not going to split hairs over a two-cent difference.

English equivalent: To split hairs

Origin: First half of the 20th century, and exactly as coarse as it looks. The point is scale: performing the act named on something the size of a fly would demand an impossible fineness of execution, so an enculeur de mouches — politely a sodomiseur de diptères — is a person whose eye for detail has gone well past the point of any use. It sits alongside pinailler, which means the same without the anatomy. The date is hard to fix; the usual citation is 1949, though tormenting flies for amusement is a much older idea — Victor Hugo has a character in César Birotteau (1837) killing flies alone at home for his own entertainment, as the emperor Domitian was said to.