Flogging a dead horse
Flogging a dead horse (alternatively beating a dead horse, or beating a dead dog 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".
Earlier meaning 
Some scholars claim that the phrase originated in 17th-century slang, where a "dead horse" was work that was paid for in advance, e.g. "His land 'twas sold to pay his debts; All went that way, for a dead horse, as one would say." This attribution confuses "flogging a dead horse" with an entirely different phrase: "to work (for) the dead horse". This phrase was slang for "work charged before it is executed". This use of 'dead horse' to refer to pay that was issued before the work was done was an allusion to using one's money to buy a useless thing (metaphorically, "a dead horse"). Most men paid in advance apparently either wasted the money on drink or other such vices, or used it to pay outstanding debts. In his book 'Old England and New Zealand' published in London in 1879 the author Alfred Simmons gives detailed explanation and background of the "Flogging the Dead Horse" ceremony performed on board ship by a ships crew at the end of the first month of their voyage (page 113).
"To slay the slain" 
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.
To twice slay the slain,
By dint of the Brain,And so I shall bid you 'Adieu'!
(Thus Huxley concludes his review)
Is but labour in vain,
Unproductive of gain,—"Monkeyana" from Punch, May 1861
- 1872 The Globe, 1 Aug 1872
- Nicker Nicked in Harl. Misc. (Park) II. 110 (1668)
- "Antigone: Tiresias' Monologue". Monologue Archive. Retrieved 2011-04-26.
- Libanius, " Julian the Emperor" (1888). Monody: Funeral Oration for Julian The Tertullian Project
- ""Monkeyana", from ''Punch'', May 1861". THE HUXLEY FILE. Retrieved 2011-04-26.
Further reading 
- Isil, Olivia A. (1996). When a Loose Cannon Flogs a Dead Horse There's the Devil to Pay: Seafaring Words in Everyday Speech. International Marine. ISBN 0-07-032877-3.
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