bee in her bonnet

Bee in her bonnet is an old cliche which is referring to being preoccupied or obsessed with an idea. If you think of it literally, a bee in someones bonnet would make them obsessed with getting the bee out of their bonnet, where they are not able to do or think about anything else. Figuratively, this phrase is used when someone is so preoccupied with one thing, they cant concentrate on anything else.