backside

(noun) the side of an object that is opposite its front. Synonyms: rear, back end. Examples: "His room was toward the rear of the hotel."

(noun) the fleshy part of the human body that you sit on. Synonyms: buttocks, nates, arse, butt, 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. Examples: "He deserves a good kick in the butt." "Are you going to sit on your fanny and do nothing?."