You're beating a dead horse: This idiom means to waste time doing something that has already been attempted or to bring up an issue that has already been concluded so you're seen as wasting your time. I have definitely heard this idiom a lot growing up. Most of the time I heard it used when people are in an argument and one person just couldn't get the other to see their point of view and they would refer to their altercation as beating a dead horse because it's redundant. If a horse is already dead their is no use to beat it. Just like when in an argument, if a person is not understanding your point of view then there is no point in trying to make them understand because repeating how you feel over and over won't make them change their mind.