bee in one's bonnet

A chronic preoccupation, often fanciful or eccentric: “My cousin has a bee in his bonnet about the rudeness of local cabdrivers; he's written four letters to the editor on the subject.”

A strange idea or notion; also, an idea that is harped on, an obsession. For example, Bill's got a bee in his bonnet about burglars; he's always imagining strange noises. This term, which replaced the earlier have bees in one's head, transfers the buzzing of a bee inside one's hat to a weird idea in one's head. [Second half of 1600s]