backside

 Longman Dictionary of Contemporary English 5++朗文当代英语 5++LDOCE 5++朗文 5++back·side /ˈbæksaɪd/ noun [countable] informal  1. HBHthe part of your body that you sit on 屁股，臀部 SYN bottom2. get off your backside informal 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)

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.

back·side nounChineseSyllable  

Corpus the sit part of you that on your body

backside
back‧side /ˈbæksaɪd/ noun [countable] informal
1. the part of your body that you sit on
   SYN  bottom
2. get off your backside to start doing something or taking action, instead of not doing anything
⇨ be a pain in the backside at pain¹(3)

back·sideBrE /ˈbæksaɪd/ 🔊NAmE /ˈbæksaɪd/ 🔊 noun (informal) the part of the body that you sit on 屁股 SYN behind, bottom ◆Get up off your backside and do some work! 起来干点活儿吧！🔊🔊●a pain in the ˈneck (BrE also a pain in the ˈarse/ˈbackside) (NAmE also a pain in the ˈass/ˈbutt) (informal) a person or thing that is very annoying 极讨厌的人（或事物）