The idiom "beat a dead horse" is believed to have originated in the 1800s, likely from horse racing. Back then, a horse that had died during a race was still sometimes whipped in an attempt to make it move and win the race. This was, of course, pointless, as the horse was already dead.
The idiom itself is meant to describe a pointless or futile activity. It emphasizes the act of continuing to work on something that is already finished, unproductive, or unchangeable.
Here are some possible explanations for the origin of the idiom:
* Horse Racing: As mentioned above, the act of whipping a dead horse during a race is a clear and concrete example of a futile activity.
* Agriculture: In the past, farmers might have beaten a dead horse to try and revive it. This was a common practice, but it was always futile.
* Metaphorical Usage: The idiom might have also evolved from a metaphorical understanding of the concept. For example, someone might say, "We've been beating this dead horse for weeks. It's time to move on."
No matter the exact origin, the idiom "beat a dead horse" has been used for centuries to highlight the futility of continuing with an unproductive task.