beat a dead horse



Definition: To keep talking about something after everything useful has already been said; to discuss a topic after it has been completely exhausted

Example: 1) We've already decided what we're going to do, so let's not talk about it anymore -- there's no reason to beat a dead horse

Marty kept beating a dead horse in the sales meeting, and it kept us there an extra half hour

Etymology: A dead horse cannot be brought back to life, even if you beat it. So if you are 'beating a dead horse', you are wasting your time.