backside
back·side / ˈbækˌsaɪd ; ˈbæksaɪd /noun Cinformal
1. the part of your body that you sit on屁股，臀部;SYNbottom
2. get off your backside to start doing something or taking action, instead of not doing anything起来干点事情
⇒be a pain in the backside at PAIN¹ (3)