From WordNet (r) 3.0 (2006) [wn]:
keister
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]