"Bee in one's bonnet" is a phrase that refers to a persistent or obsessive idea that someone can't seem to shake off. There are many synonyms that can be used interchangeably with this phrase, such as "obsession," "preoccupation," "fixation," "mania," "a bugbear," "an idee fixe," "a pet project," or "a bee in one's bonnet." These synonyms all describe an idea or problem that a person is fixated on and can't seem to let go of, and it can often lead to frustration or annoyance for others. So whether it's an idea that you can't stop thinking about or a task that nags at you constantly, sometimes it's better to let go and move on.