backside
noun /ˈbæksaɪd/
  /ˈbæksaɪd/
 (informal)Idioms - the part of the body that you sit on synonym behind, bottom
- Get up off your backside and do some work!
 
 
Idioms 
a pain in the backside/arse (British English also) 
(North American English also a pain in the ass/butt)
 (also a pain in the neck)
- (informal) a person or thing that is very annoying
- That man's a pain in the backside!