live rent-free in (one's) head

slang Of a person, to be a source of antipathy or exasperation to one to the extent of becoming a frequent or continual subject of one's agitated thoughts. The term indicates that such a person is able to negatively impact one without any repercussions (with the implication that one should not allow such a person to have such an effect on them). Don't let your opponent live rent-free in your head, man. Shake it off! My mom has been living rent-free in my head for years—that's why I would deny my own wants and needs and instead do things that I thought would make her happy.