1540s, from Middle English garce (early 13c.), from Old North French garser "to scarify, cut, slash" (Old French *garse), apparently from Vulgar Latin *charassare, from Greek kharassein "engrave," from PIE *gher- "to scrape, scratch" (cf. character). Loss of -r- is characteristic (see ass (n.2)). Slang use for "vulva" dates to mid-1700s.
1560s, alteration of garsen (late 14c.), from Old North French garser "to cut, slash" (see gash (n.)). Related: Gashed; gashing.