1
general::
Also off someone's case. No longer harassing or bothering someone. It is often put as get off someone's back or case, as in I told her to get off my back? I'll mow the lawn tomorrow, or I wish Dad would get off my case about grades. The first of these slangy terms dates from the 1880s although it became frequent only in the 1940s, and its antonym, on one's back (as in He's been on my back about that report all morning) dates from about 1960. The variant off someone's case was first recorded only in 1970, and its antonym, on someone's case (as in He's always on my case) in 1971. Also see GET OFF, def. 8.
American Heritage Idioms