be flogging a dead horse
To be continuing to focus on something, especially an issue or topic, that is no longer of any use or relevance. I don't know why you're flogging a dead horse—the rest of us have moved on from that problem.
be flogging a dead horse
BRITISH or be beating a dead horse
AMERICANIf 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.