'Backside' definitions:

Definition of 'backside'

(from WordNet)
noun
The side of an object that is opposite its front; "his room was toward the rear of the hotel" [syn: rear, backside, back end] [ant: forepart, front, front end]
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?" [syn: buttocks, nates, arse, butt, backside, 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]

Definition of 'Backside'

From: GCIDE
  • Backside \Back"side`\ (b[a^]k"s[imac]d`), n. [Back, a. + side.] The hinder part, posteriors, or rump of a person or animal. [1913 Webster]
  • 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. [1913 Webster]