WordNet sense 1 (the fleshy part of the human body that you sit on):

backside, behind, bottom, bum, buns, buttocks, can, derriere, fanny, hindquarters, rear, rump, seat, stern, tail, tush, arse, fundament, keister, nates, prat, tooshie, tush, tail end, hind end

WordNet sense 2 (a tooth situated at the back of the mouth):

back tooth

WordNet sense 3 (situated in or directed toward the part of the body from which the tail arises):

caudal

WordNet sense 4 (located at or near the back of an animal):

back, hind, hinder, hinder