What does "clean someone's clock" mean — and why is it funny?

informal, American

Meaning

To beat someone decisively — either in a physical fight or, more loosely, in a contest or argument.

Where it comes from

American slang from the early 1900s. 'Clock' had long been slang for a person's face — the thing you 'read' to tell what someone is thinking — and 'clean' carried the sense of a thorough, complete job.

Why it is funny

The humor is the collision of two utterly unrelated worlds. 'Clean' belongs to tidying and chores; a fist to the face does not. The phrase dresses up a beating in the calm language of housework, as if knocking someone flat were just a tidy bit of maintenance. Add the already-comic idea of a face being a 'clock', and you get violence described with a polite, almost domestic straight face.

Used in a sentence

"He talked tough before the match, but the champion cleaned his clock in the first round."