The Collaborative International Dictionary of English v.0.48:

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]

WordNet (r) 3.0 (2006):

backside
    n 1: 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]
    2: 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]