backside
noun.
the side of an object that is opposite its front
his room was toward the rear of the hotel
Synonym: back end, rear
noun.
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?
Synonym: arse, nates, posterior, prat, rear, rear end, rump, seat, stern, tail, tail end, tooshie, tush, keister, hindquarters, ass, behind, bottom, bum, buns, butt, buttocks, can, derriere, fanny, fundament, hind end