To Beat a Dead Horse
Tratar de Revivir a un Muerto
Meaning:
You beat a dead horse when you are trying to get an impossible goal, when you continue fighting a lost battle or keep arguing after a final decision has been made. A similar expression is to try to get water out of a dry well.
Origin:
In ancient Rome, a playwright Plautus used this expression in one of his plays in the year 195 B.C. In it, a farmer whipped a dead horse so as to make it move a heavy load. This situation teaches us that sometimes we try to get worthless pursuits.
Example:
I did not study Mathematics at all along the year and one week before the term exam was given, I tried to cram all that knowledge into my mind. In the end, I did not pass the test. Everything I did was useless. It was like beating a dead horse.