The Collaborative International Dictionary of English v.0.48:

buttocks \but"tocks\ n. pl.
   the fleshy part of the human body at the back of the hip, on
   which a person sits.

   Syn: 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.
        [WordNet 1.5]

WordNet (r) 3.0 (2006):

buttocks
    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:

37 Moby Thesaurus words for "buttocks":
   arse, ass, backside, beam, behind, bottom, breech, bum, butt, can,
   cheeks, croup, crupper, derriere, duff, fanny, fundament, haunches,
   heinie, hind end, hinder, hindquarters, keister, nates, posterior,
   prat, rear, rear end, rump, seat, secretiveness, stem, tail,
   tail end, tuchis, tush, tushy