<<Flogging a dead horse (alternatively beating a dead horse in some parts of the Anglophone world) is an idiom that means a particular request or line of conversation is already foreclosed or otherwise resolved, and any attempt to continue it is futile; or that to continue in any endeavour (physical, mental, etc.) is a waste of time as the outcome is already decided.
The first recorded use of the expression with its modern meaning is by British politician and orator John Bright, referring to the Reform Act of 1867, which called for more democratic representation in Parliament, an issue about which Parliament was singularly apathetic. Trying to rouse Parliament from its apathy on the issue, he said in a speech, would be like trying to flog a dead horse to make it pull a load. The Oxford English Dictionary cites The Globe, 1872, as the earliest verifiable use of flogging a dead horse, where someone is said to have "rehearsed that [...] lively operation known as flogging a dead horse".
A comparable expression for useless labour is "thrice to slay the slain", a quotation from John Dryden, in Alexander's Feast, stanza iv. Dryden drew his inspiration from Sophocles' Antigone in which the blind seer Tiresias is led onstage by a boy, and declaims, "Nay, allow the claim of the dead; stab not the fallen; what prowess is it to slay the slain anew?" The trope was used in Latin, too: in Libanius' funeral oration for the Emperor Julian, he declares of a scoundrel, "Of the three who had enriched themselves through murders, the first had gone over the whole world, accusing people falsely, and owed ten thousand deaths to both Europe and Asia; so that all who knew the fellow were sorry that it was not possible to slay the slain, and to do so thrice over, and yet oftener." The expression was used in "literary" contexts, as when Edward Young mused: "While snarlers strive with proud but fruitless pain To wound immortals, or to slay the slain."
In the heated atmosphere of literary journalism, the phrase was often quoted to show the writer's knowledge. In Punch for May 1861, a broad
satire on the heated controversies caused by the publication of Charles Darwin's Origin of Species, which was defended by Thomas Henry
Huxley, concluded as follows.
By dint of the Brain,
(Thus Huxley concludes his review)
Is but labour in vain,
Unproductive of gain,
And so I shall bid you 'Adieu'!
—"Monkeyana" from Punch, May 1861>>