From Longman Dictionary of Contemporary Englishbacksideback‧side /ˈbæksaɪd/ noun [countable] informal 1 HBHthe part of your body that you sit on SYN bottom2 → get off your backside → be a pain in the backside at pain1(3)
Examples from the Corpus
backside• Many of them needed some kind of kick up the financial backside.• Mid-forties and creaking with crime-fatigue and cynicism, his backside hits the bed hard.• Somehow he had landed on his backside and then had felt his tailbone knock the earth harder than was safe.• Now we were both on our backsides in the mud. 