Beating a dead horse

Some examples of beating a dead horse include:

• Going after an ex-girlfriend after she's already started dating someone else, called you several dirty names, and threatened to call the cops because she thinks you're stalking her. Going on after that point would be like beating a dead horse.
• Explaining something to someone who doesn't care anyway but hasn't given you the favorable response you were looking for in the first place.
• Attempting to convert an aetheist to Christianity, or anything for that matter.
• Stating your opinion about how XP and reputation and voting in E2 and all that works, when you notice that every time you post something like that you get downvoted into the lower pits of Hell. No one cares. No one's listening to you. It's like beating a dead horse.
• Coming up with examples of beating a dead horse long after you've run out of ideas.

In the days of sailing ships, merchant seamen were often given an advance on their first month's wages in order to purchase clothing and other essentials before a long voyage. Then for the first month they received no wages. They were said to be "working a dead horse," meaning they were working for no pay. After the month was over, a piece of canvas cut into the shape of a horse was hoisted up into the rigging to signify that from that day forward all seamen would be receiving wages. Beating a dead horse means to do something for nothing.