WordNet (r) 3.0 (2006):

rear end
    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]

Moby Thesaurus II by Grady Ward, 1.0:

33 Moby Thesaurus words for "rear end":
   afterpart, afterpiece, back, back door, back seat, back side,
   backside, behind, bottom, breech, butt, buttocks, croup, crupper,
   fanny, haunches, heel, hind end, hind part, hindhead, nates,
   occiput, posterior, postern, rear, rearward, reverse, rump, seat,
   stern, tail, tail end, tailpiece