The Collaborative International Dictionary of English v.0.48:

buns \buns\ n. pl.
   the buttocks. [Slang]

   Syn: buttocks, arse, butt, backside, bum, 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):

buns
    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]