What does it mean when someone says you are talking to a dead horse?

When someone says you are "talking to a dead horse," they mean that you are wasting your time trying to convince someone of something or discussing a topic that has already been settled or is unlikely to change. It's an idiom that suggests futility in continuing the conversation, as the issue is effectively "dead" and cannot be revived. Essentially, it's a way of saying that further discussion is pointless.