Definition: to belabor a point; to insist on talking about something that cannot be changed
My boss hated my idea, and he said “no” quickly. But I continued to argue with him, and push my idea. I finally stopped, because I was wasting my breath. I was beating a dead horse.
I understood you the first time. You don’t need to keep saying the same thing, over and over and over. Stop beating a dead horse.