"buttocks," Old English ærs "tail, rump," from Proto-Germanic *arsoz (cognates: Old Saxon, Old High German, Old Norse ars, Middle Dutch ærs, German Arsch "buttock"), from PIE root *ors- "buttock, backside" (cognates: Greek orros "tail, rump, base of the spine," Hittite arrash, Armenian or "buttock," Old Irish err "tail"). Middle English had arse-winning "money obtained by prostitution" (late 14c.).