Flog a dead horse

Pro Tip 1/3

Wasted Effort

The phrase implies that any effort put into the task is wasted and pointless.

He's still fixing that old car, but it's like flogging a dead horse.

Pro Tip 2/3

Futile Argument

Often used to suggest that arguing or persuading is useless because the decision is already made.

Don't bother arguing with her about the trip; it's flogging a dead horse.

Pro Tip 3/3

Move On

Suggests that it's time to stop an action and proceed to something else more productive.

Stop revising the proposal, it's just flogging a dead horse now.