fundament

backside, behind, booty (also bootie) [slang], bottom, breech, bum, buns, butt, buttocks, caboose, can, cheeks, derriere (or derrière), duff, fanny, hams, haunches, heinie [slang], hunkers, keister (also keester) [slang], nates, posterior, rear, rear end, rump, seat, tail, tail end, tush [slang]