What does "a bee in your bonnet" mean — and why is it funny?

informal

Meaning

To be preoccupied or obsessed with a single idea, and to keep bringing it up.

Where it comes from

A British phrase built from an older image. A 'bonnet' is a hat, and a trapped bee buzzing inside it is impossible to ignore.

Why it is funny

The humor is the frantic little image. An obsession becomes an actual bee, loose under your hat, buzzing away until you can think of nothing else — which is exactly how a fixed idea feels.

Used in a sentence

"He's got a bee in his bonnet about recycling and won't stop talking about it."