I always thought it was Anglo Saxon. Most four letter 'swear' words are. However, it doesn't have a hard vowel sound as the rest of them do, so maybe I'm wrong.
Actually it's Old English:-
buttocks," O.E. ærs "tail, rump," from P.Gmc. *arsoz (cf. O.N. ars, M.Du. ærs, Ger. Arsch "buttock"), cognate with Gk. orros "tail, rump, base of the spine," Hittite arrash, Arm. or "buttock," O.Ir. err "tail." A--e-hole first attested c.1400 as arce-hoole. Arsy-versy "backside foremost" first attested 1539.