From WordNet (r) 2.0 (August 2003) [wn]:
keister
n : 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, posterior,
prat, rear, rear end, rump, stern, seat,
tail, tail end, tooshie, tush, bottom,
behind, derriere, fanny, ass]