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on (one's) case
Constantly nagging, harassing, or annoying someone. My boss has been on my case all week to get that report done. I'm sorry for getting on your case about it, but I need an answer soon.
*on someone's caseand *on someone's back
Inf. harassing someone about a personal problem; annoying someone. (*Typically: be ~; get ~; keep ~.) I'll get on Tom's case about being late so much. I'm sorry, I won't get on your case anymore.
on (someone's) case
Persistently nagging or urging someone to do something.