It is useless to flog a dead horse

It is a waste of time to try to revive interest in a worn-out or forgotten subject, or to argue about something that was settled long since.

‘I said it at the time and I say it again that when they built the new bypass they should have taken it round the north of the town, not round the south.’

‘Why bring that up again? Nothing can be done about it now. You’re flogging a dead horse.’

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