From WordNet (r) 3.0 (2006):
rump
n 1: the part of an animal that corresponds to the human
buttocks [syn: hindquarters, croup, croupe, rump]
2: fleshy hindquarters; behind the loin and above the round
3: 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]