What does "mad as a box of frogs" mean — and why is it funny?

informal, British

Meaning

Completely crazy, in a chaotic and usually harmless or endearing way.

Where it comes from

A relatively modern British phrase, built on the simple, vivid picture of frogs trapped together in a box.

Why it is funny

The humor is the wonderfully specific chaos. Picture a sealed box of frogs — all leaping, bumping and scrambling in every direction at once — and you have a perfect, jittery image of a delightfully scrambled mind.

Used in a sentence

"She wants to repaint the whole house overnight — mad as a box of frogs."