It's Still Money

Is it "petty cash" or "petit cash"? Why? Please answer soon. —Having a Petty Argument

Dear HPA,

According to the OED and a few editions of Webster's American, it's petty cash. Why? I guess because it would seem absurdly pretentious to write a lofty French petit next to a crass late-16^th-century English word (although both "petty" petit and "cash" caisse come from French). However, if you Google it, you'll find "petit cash."