Flog a dead horse

Definitions

Waste time and effort
To continue to work on a project or task that is already doomed to failure, instead of accepting the inevitable and moving on.
Persist in a futile effort
To keep trying to achieve something that is impossible or unrealistic, despite all evidence and logic indicating that it is a lost cause.
Rehash old arguments or issues
To bring up a topic or debate that has already been resolved or is no longer relevant, causing frustration and annoyance to others.

Examples of Flog a dead horse

Trying to sell a product that has already been discontinued is like flogging a dead horse.

This idiom means to persistently and futilely pursue something that is no longer viable or productive. In this example, it is comparing the act of trying to sell something that is no longer in production to the futility of whipping a dead horse. It implies that the effort will yield no positive results.
The sales team kept pitching their failed product to potential clients, even though it had already bombed in the market. WE'RE FLAGGING A DEAD HORSE!

This idiom means trying to revive something that has already failed or doesn't have any value anymore. In this example, the sales team is wasting their time and resources by continuing to promote a product that has already failed in the market. It's a futile effort, similar to flogging (beating) a dead horse, which is a senseless activity as a dead horse cannot respond. The sales team is wasting their effort on a lost cause, just like flogging a dead horse.
The manager insisted on implementing the same marketing strategy that failed last year, instead of researching and developing new ideas. FLAGGING A DEAD HORSE!

This idiom is used when someone repeats a failed strategy or decision, despite clear evidence of its failure. In this example, the manager is persisting with the same marketing strategy that failed last year. It's similar to flogging a dead horse because the strategy is not going to work any better now than it did last year. The manager is repeating the same mistake, which is a waste of time and resources.
The writer kept trying to force a subplot into his story, even though it didn't fit with the main plot. FLAGGING A DEAD HORSE!

This idiom is used when someone tries to force an idea, concept, or subplot into a larger piece of work or project, even if it doesn't fit or add any value. In this example, the writer is attempting to introduce a subplot into his story, which doesn't fit with the main plot. This is similar to flogging a dead horse because it's a futile effort, much like beating a dead horse. The writer is trying to revive an idea that doesn't have any value, and it's a waste of time and resources.
The chef kept serving the same dish, even after it received poor reviews from customers. FLAGGING A DEAD HORSE!

This idiom is used when someone continues to serve or promote a dish or menu item, despite negative reviews or customer feedback. In this example, the chef is still serving the same dish, despite poor reviews from customers. This is similar to flogging a dead horse because it's a pointless exercise, much like beating a dead horse. The chef is attempting to revive a dish that customers have already rejected. It's a waste of time, resources, and ingredients.
Trying to persuade your boss to invest in a failing company is like flogging a dead horse.

This idiom means continuing to pursue a lost cause, insisting on something that's no longer productive, and wasting your time and energy. Here, we're comparing it to futilely trying to get a dead horse to move by whipping it.
Debating with your friend about a topic that's long been resolved is like flogging a dead horse.

This idiom means persisting in a discussion or argument that's already finished and settled. Here, we're comparing it to chasing after a lifeless animal by whipping it, in the hopes that it will magically begin to move again.
Working on a project that's already past its due date is like flogging a dead horse.

This idiom means attempting to complete a task that's severely overdue, having missed several deadlines, and is now pointless. Here, we're comparing it to whipping a deceased animal, hoping that it will miraculously come back to life and fulfill its purpose.

Conclusion

The idiom "flog a dead horse" can be used in a variety of contexts, but all convey the idea of wasting time, effort, or resources on something that is already doomed to fail or has no chance of success. It can be used to discourage someone from pursuing a particular course of action or to criticize someone for being stubborn or unyielding in their efforts.

In the first usage, the idiom suggests that the person is stubbornly continuing to work on a project or task that is already doomed to failure, instead of accepting the inevitable and moving on. This could be due to a lack of acceptance or understanding of the situation, or simply an unwillingness to give up.

In the second usage, the idiom implies that the person is persisting in a futile effort, despite all evidence and logic indicating that it is a lost cause. This could be due to a sense of determination or stubbornness, or a refusal to admit defeat.

In the final usage, the idiom suggests that the person is rehashing old arguments or issues, causing frustration and annoyance to others. This could be due to a lack of awareness or sensitivity to the feelings of others, or a desire to cling onto past grievances.

Origin of "Flog a dead horse"

The origin of this idiom is unclear, but it is believed to have originated in the 17th or 18th century. It may have originated from the practice of beating a dead horse as a form of punishment for a lazy or disobedient worker. In this context, "flog" means to beat or whip.

The phrase became popularized in English literature in the 19th century, appearing in works by authors such as Charles Dickens and Mark Twain. It is now widely used in everyday language to convey the idea of wasting time and effort.