"Bee in the bonnet" is a colloquial expression which means a persistent and obsessive idea or thought. There are many synonyms that can be used instead of this phrase depending on the context and the level of intensity of the idea. Some of the synonyms for "bee in the bonnet" include "fixation," "obsession," "compulsion," "mania," "preoccupation," "possession," "persistence," and "nagging feeling." These words can be used interchangeably to convey the same meaning as "bee in the bonnet." Choosing the right synonym depends on the tone and the message the speaker wants to convey.