The Collaborative International Dictionary of English v.0.48:

arsehole \arse"hole`\ ([aum]rs"h[=o]l`), n.
   1. execretory opening at the end of the alimentary canal.

   Syn: anus, arse, asshole
        [WordNet 1.5]

WordNet (r) 3.0 (2006):

arsehole
    n 1: vulgar slang for anus [syn: arse, arsehole, asshole,
         bunghole]