get off (one's) rear
slang To start doing something, especially after a period of unproductivity or laziness. Grandma will be here in 30 minutes, so get off your rear and move this junk out of the living room! Ted needs to get off his rear for a change and make an effort around here.
get off one’s rear
in. to get up and get busy. (Euphemistic for Get off my ass!) It’s time to get off your rear and get to work.