[n] the fleshy part of the human body that you sit on
[n] the side of an object that is opposite its front; "his room was toward the rear of the hotel"
arse, ass, back end, behind, bottom, bum, buns, butt, buttocks, can, derriere, fanny, fundament, hind end, hindquarters, keister, nates, posterior, prat, rear, rear, rear end, rump, seat, stern, tail, tail end, tooshie, tush
\Back"side`\, n. [Back, a. + side. ] The hinder part, posteriors, or rump of a person or animal. Note: Backside (one word) was formerly used of the rear part or side of any thing or place, but in such senses is now two words.