be flogging a dead horse

To be continuing to focus on an issue or topic that is no longer of any importance or relevance. I don't know why you're flogging a dead horse—the rest of us have moved on from that problem. OK, Tom, you've made your point. Now you're just flogging a dead horse. Come on, I said I was sorry! Why are you still flogging a dead horse?

be flogging a dead horse

BRITISH or

be beating a dead horse

AMERICAN

If someone is flogging a dead horse, they are wasting their time trying to achieve something that cannot be done. After putting in all that hard work it feels like we're flogging a dead horse — it's all very discouraging. You're beating a dead horse on this. These guys are defeated.