a kick up the backside

A forceful gesture or message of some kind (usually delivered with good intentions) that acts as motivation to the (previously unmotivated) recipient. Primarily heard in UK, Australia. The threat of divorce was the kick up the backside he needed to start working on improving his marriage.

COMMON If someone or something gives someone a kick up the backside, they shock them into trying harder to do something. I hope this gives him the kick up the backside he needs. Maybe he'll try harder to look for a job now. Note: This expression is often varied. For example, in British English up the arse or in the pants can be used instead of up the backside, and in American English in the ass can be used instead of in the butt. He got four goals. But he needed a kick up the arse before he started to play properly. Note: You can also say that a person or event kicks someone up the backside or in the pants. Did you think universities needed kicking up the backside a little bit? He sometimes had to be kicked in the pants to make full use of his athletic gifts.