Source : Webster's Revised Unabridged Dictionary (1913)

Backside \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.

Source : WordNet®

backside
     n 1: the side of an object that is opposite its front; "his room
          was toward the rear of the hotel" [syn: {rear}, {back
          end}] [ant: {front}]
     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}, {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}]