backside

nouns

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