The Collaborative International Dictionary of English v.0.48:

Arse \Arse\ ([aum]rs), n. [AS. ears; [ae]rs; akin to OHG. ars.
   G. arsch, D. aars, Sw. ars, Dan. arts, Gr. ? (cf. o'yra`
   tail).]
   The buttocks, or hind part of an animal; the posteriors; the
   fundament; the bottom.
   [1913 Webster]

WordNet (r) 3.0 (2006):

arse
    n 1: the fleshy part of the human body that you sit on; "he
         deserves a good kick in the butt"; "are you going to sit on
         your fanny and do nothing?" [syn: buttocks, nates,
         arse, butt, backside, bum, buns, can,
         fundament, hindquarters, hind end, keister,
         posterior, prat, rear, rear end, rump, stern,
         seat, tail, tail end, tooshie, tush, bottom,
         behind, derriere, fanny, ass]
    2: vulgar slang for anus [syn: arse, arsehole, asshole,
       bunghole]