flog a dead horse

flog a dead horse
also, beat a dead horse

Meaning:

• waste energy on something that has no chance of succeeding
• waste effort on a lost cause
• engaging in a fruitless effort
• attempting to revive interest in something which has died out

Example:

1. He keeps trying to get his manuscripts published, but I think he is flogging a dead horse.
2. There's no use trying to keep this business going. We are flogging a dead horse.
3. I've told him numerous times to manage his routine effectively, but I think I've just been flogging a dead horse.
4. You're flogging a dead horse trying to persuade him to come with us – he hates going out at night.
5. You keep trying to convince him to join your company, but I think you're flogging a dead horse. He's happy where he is.
6. The teacher was flogging a dead horse when he lectured the students about the dangers of too much technology – they were glued to their smartphones while he spoke.

Origin:
The phrase first appeared in print in 1859 in a report of a UK parliamentary debate.