[beat] a dead horse
1
to waste energy on a lost cause
The origin of the idiom "beat a dead horse" comes from the practice of beating a dead horse in order to make it move, which is obviously futile since the horse is already dead. Today, the idiom is commonly used in a figurative sense, to describe someone who is continuing to pursue a topic or argument that has already been resolved or has lost its relevance. The idiom implies that further effort or discussion is pointless and that it's time to move on to something else.
- We've gone over this plan a dozen times—there's no point beating a dead horse.
- I don't want to beat a dead horse, but the project is definitely over budget.
- Asking for another refund now is like beating a dead horse; the policy won't change.
- She realized she was beating a dead horse when her idea was flatly rejected.
- Trying to fix that ancient software now feels like beating a dead horse.