What is the origin of the idiom "beat a dead horse" and when to use it?

The origin of the idiom "beat a dead horse" comes from the practice of beating a dead horse in order to make it move, which is obviously futile since the horse is already dead. Today, the idiom is commonly used in a figurative sense, to describe someone who is continuing to pursue a topic or argument that has already been resolved or has lost its relevance. The idiom implies that further effort or discussion is pointless and that it's time to move on to something else.