The phrase "bee in one's bonnet" is an idiomatic expression that means having a particular idea or obsession that dominates one's thoughts and actions. It suggests that someone has a strong, often irrational, belief or fixation on something, which can cause them to behave in a way that may seem strange or unreasonable to others. The phrase originated from the idea of having a bee buzzing around inside your head, causing you to feel agitated and distracted by its constant presence.