arse

First recorded before 1000; Middle English ars, er(e)s, Old English ærs, ears; cognate with Old Frisian ers, Dutch aars, Old Norse, Middle Low German, Old Saxon, Old High German ars (German Arsch ), Greek órrhos “rump” (from orso-, which occurs in compounds), Armenian or̄kh, Hittite arras; akin to Greek ourá and Old Irish err “tail.” All of the nouns derive from the Proto-Indo-European root ers-, ors- “backside, buttocks, tail.” Cf. ass²

In American English, ass in the senses "the buttocks; the rectum" is considered to be a vulgar, impolite usage. The variant arse , which is chiefly British and regarded as vulgar by speakers of British English, seems to some American speakers to be a less vulgar, even daintier term than the more explicit ass . But this may not always be the case in America, so one should be just as cautious about using arse when in a polite setting.