Synonyms for Arse (same or very similar meaning)
WordNet sense 1 (the fleshy part of the human body that you sit on):
backside, behind, bottom, bum, buns, buttocks, can, derriere, fanny, hindquarters, posterior, rear, rump, seat, stern, tail, tush, fundament, keister, nates, prat, tooshie, tush, tail end, hind endWordNet sense 2 (excretory opening at the end of the alimentary canal):
assholeFrom the ODE community, based on WordNetadd/edit