From Longman Dictionary of Contemporary Englishbe on somebody’s casebe on somebody’s caseinformalCRITICIZE to be criticizing someone constantly Dad’s always on my case about something or other. → case
Examples from the Corpus
be on somebody’s case• Dad's always on my case about getting a job.• First you are on this case, then you are off.• In fact there was not much of a feeling that anyone was on my case at all.• And the National Guard were on the case now.• I dreamed I was on a case.• But Jack MacFarland was on the case.• Around 50 police officers are on the case.• And Rathbone would be on the case, with his bloodhound and magnifying glass. 