From 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]