the.com/bee in the bonnet

means To have a fixed, often obsessive idea or preoccupation that you keep returning to, usually one others find a bit much.

from The image comes from Scots verse — Robert Herrick wrote of a 'bee in the head' in the 1600s meaning a wild or restless whim, and by the 1700s Scottish and English writers had moved the bee into a bonnet. The logic is simple: imagine an actual bee trapped under the brim of your hat, buzzing endlessly against your skull — impossible to ignore, impossible to evict. That frantic, inescapable buzzing became the perfect shorthand for an idea you can't shake.