definition of
backside

[1] #1 backside n. [rear, back end] the side of an object that is opposite its front; "his room was toward the rear of the hotel". wn

[2] #2 backside n. [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] 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?". wn