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 particular idea or notion, esp a fantastic or eccentric one; obsession: He's got a bee in his bonnet about wheat germ curing all the world's ills
[mid-1800s+; attested as bee in one's head fr early 1500s]