Princeton's WordNet

1. rear, backside, back end(noun)

   the side of an object that is opposite its front

   "his room was toward the rear of the hotel"

2. 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(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?"

Wiktionary

1. backside(Noun)

   The side of something that is opposite the front.

   The backside of the building faced an alley, and was covered in grime and scrawled graffiti.

2. backside(Noun)

   The buttocks.

   After riding the horse all day for the first time, I had painful blisters on my backside.

Webster Dictionary

1. Backside(noun)

   the hinder part, posteriors, or rump of a person or animal

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