backside

n. butt, rear-end, buttocks

Frontside and backside are surfing, skateboarding, snowboarding and Aggressive Inline Skating terms that are used to describe how a person approaches an obstacle or performs a certain trick. In Aggressive Skating, frontside and backside are types of grinds.The names frontside and backside originate from surfing where they mean the direction the surfer is facing while surfing a wave. If the surfer is facing the wave, he or she is surfing frontside, otherwise he or she is surfing backside. The terms forehand and backhand are synonyms for frontside and backside but they are only used in surfing.
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Noun
1. the side of an object that is opposite its front; "his room was toward the rear of the hotel"
(synonym) rear, back end
(hypernym) side, face
(hyponym) nape, scruff, nucha
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?"
(synonym) 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
(hypernym) body part
(part-holonym) torso, trunk, body

(n.)
The hinder part, posteriors, or rump of a person or animal.
  

buttocks, bum, buns When I was learning to skate, I fell a lot. My backside was sore!