WordNet (r) 3.0 (2006):

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]

Moby Thesaurus II by Grady Ward, 1.0:

18 Moby Thesaurus words for "keister":
   arse, ass, behind, bottom, bum, can, cheeks, duff, fanny,
   posterior, prat, rump, rusty-dusty, stern, tail, tuchis, tush,
   tushy