What does "flogging a dead horse" mean — and why is it funny?

informal

Meaning

Wasting effort on something that has no chance of success.

Where it comes from

A British phrase popularised in the 1800s. To 'flog' a horse is to whip it onward, and a dead one is going precisely nowhere.

Why it is funny

The humor is grim and absurd at once. You picture someone whipping away with great determination at a horse that is plainly, completely past caring.

Used in a sentence

"Asking him to apologise is flogging a dead horse."