I think this may be more of a literary question than a horse one...
I believe it refers to "flogging a dead horse" which it a way of saying you are trying to get get results when the issue has been resolved or dropped.
"When the horse is dead, get off" means you should not waste time on things that are not worth it. If the horse is dead, you can't ride it any more.
It means that you should stop talking about the subject because it's useless and already been talked about enough. (A dead horse won't do anything for you, no matter how many times you beat it.)
It's a useless action. (A dead horse won't pull your wagon no matter how much you beat him.)
The actual quote is "stop beating a dead horse" stop wasteing time on a pointless activity.
- My efforts are futile - Pissing in the wind (Australian slang) etc. bark at the moon catch at shadows bite a file beat the air in vain plough the air etc.
The Horse Is Dead was created in 1996.
No good with a dead horse.
Usually "flogging a dead horse" imagine you are beating the horse trying to get it to move even though it is dead. It means that you are trying to do something that cannot be done.
Dead lame means the horse basically can't move. It can also mean 'three legged lame' where three of the horses legs are lame.
Dead Horse - song - was created in 1991.
a dead horse
"To flog a dead horse" is the English equivalent to "to beat a dead horse", which means to do something over and over again when it is not necessary, whether it be saying something or doing something.