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]

Bouvier's Law Dictionary, Revised 6th Ed (1856):

BACKSIDE, estates. In England this term was formerly used in conveyances and
even in pleadings, and is still, adhered to with reference to ancient
descriptions in deeds, in continuing the transfer of the same. property. It
imports a yard at the back part of, or behind a house, and belonging
thereto: but although formerly used in pleadings, it is now unusual to adopt
it, and the word yard is preferred. 1 Chitty's Pr. 177; 2 Ld. Raym. 1399.