"beat a dead horse"
If someone is "beating a dead horse" it means they're talking about something that's already been answered and there's no point in talking about it again. Usually, if someone is still worrying about something that's already been agreed on, it makes other people frustrated. They'll say, "Stop beating a dead horse. Lets talk about something else." It has nothing to do with beating or with a horse. Idioms can be strange!