You are beating a dead horse when you insist on talking about something that cannot be changed. Example: "I'd like to talk with you again about what happened." Reply: "Oh, come on. Let's not beat a dead horse."

Beating a dead horse is an action that has no purpose, because no matter how hard or how long you beat a dead horse, it is not going to get up and run. Example: "Let's not talk about it any more. Okay?" Reply: "You're right. We're just beating a dead horse."

To repeatedly bring up a particular topic with no chance of affecting the outcome is beating a dead horse. Example: "Dad, are you sure we can't get a new computer for the upstairs?" Reply: "Son, we talked about this and the decision was 'no'. You are beating a dead horse."