What does "champing at the bit" mean — and why is it funny?

informal

Meaning

Extremely eager and impatient to get started.

Where it comes from

From horses. A 'bit' is the metal bar in a horse's mouth, and an impatient horse noisily chews — champs — at it before it is allowed to run.

Why it is funny

The mild humor is how horsey the image stays. The phrase turns an eager person into a restless animal, mouthing its metal bit and stamping, barely held back from bolting off.

Used in a sentence

"The kids were champing at the bit to open their presents."