BEAT A DEAD HORSE
Meaning- to pursue a useless goal.
Usage-He was trying to convince his father, but he was beating a dead horse.
Origin- This saying goes back to the ancient Roman playwright Plaitus who used in 195B.C. in one of his plays. The dramatic performance was of trying to whip a dead horse to get up and carry the load it was supposed to be moving.This action was shown to accomplish nothing. Today, we say that anyone pursing an issue that is already settled is “beating a dead horse”.