I was looking for the origin of this word and have come across two different origins:
First, from www.phrases.org.uk:
flog (or beat) a dead horse. Though he supported the measure, British politician and orator John Bright thought the Reform Bill of 1867, which called for more democratic representation, would never be passed by Parliament. 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. This is the first recorded use of the expression, which is still common for 'trying to revive interest in an apparently hopeless issue.' Bright's silver tongue is also responsible for 'England is the mother of Parliament,' and 'Force is not a remedy,' among other memorable quotations. He was wrong about the Reform Bill of 1867, however. Parliament 'carried' it, as the British say." From the "Encyclopedia of Word and Phrase Origins" by Robert Hendrickson (Facts on File, New York, 1997.)
UsingEnglish.com defines "beating a dead horse" the same way, with an actual dead horse.
However, I have also heard it came from sailors:
A dead horse was the seaman's term for the first month at sea, a month for which he was already paid and spent the money soon afterwards. To the seaman, with his money gone, he was working that first month "for free." To mark the end of this "dead horse" month, the crew would make an effigy of a dead horse, beat the thing, and dump it overboard in celebration. To officers on the ship, beating a dead horse described the difficulty in getting the crew to do any extra work during this first month at sea.
I heard this on "Says You" (a show on NPR) and later found someone posted this on some forum.
I am just wondering, what is the correct origin? Whenever someone explains what "beating a dead horse" means, they say the literal beating of a dead horse. While this explanation does describe the meaning of the saying, it may not have anything to do with its origin.
Quite often, these types of metaphors predate written language and are carried from one culture to the next for many generations before actually being recorded. It is likely that both the politician and the sailors borrowed this saying for different purposes.