A person who continues, at length to continue on a topic that has exhausted its usefulness to the rest of the crowd, to expound and explore the dead topic.
Someone who, despite being told the topic is a "dead horse" continues to converse on that topic.
I can't believe Tim is still arguing about toothpaste with Bob, Bob agreed with him 20 minutes ago! What a dead horse Jockey.
It doesn't matter how much you want to continue riding beating a dead horse is not going to get you anywhere.
If something is already done and over with there is no point in still talking about it.
If something is already broken there is no point in trying to use it.
Someone breaks up with there girl/boyfriend and continues to bitch about them to there friends weeks later. That person is beating a dead horse, the relationship is over there is no point in thinking about it since the matter is already resolved with you having broken up.
Relationship = dead horse
Bitching about it = beating it
Trying over and over to start your car after you've been in an accident where it is clear that the car is not going to work.
Broken Car = dead horse
Trying to start it = beating it
saying that means what you are asking for is absolutely not going to happen.
When somebody asks for something that you're not going to give them or do for them, your response is "Sorry, but I'd say you're looking up a dead horse's ass"