The saying “to flog a dead horse” means to try to revive interest in an issue that appears to be entirely hopeless.
“Dead horse” has long been used (for more than three centuries) as meaning something of no present value, as, “to pay, or work, for a dead horse,” to continue to pay or labor for something that no longer exists, like continuing to pay the installments on an automobile after it was smashed.
The present phrase, however, dates only to the last century. It is ascribed to the British statesman and orator, John Bright, who probably used it on at least two occasions.
One was when John, Earl Russell, sought the passage by Parliament of a reform measure, and the other was when his friend, Richard Cobden, was similarly seeking a reduction in expenditures.
Bright favored both of these measures; both had at one time interested the Parliament, but that interest had waned. It was, as Bright said, like flogging a dead horse to rouse Parliament from its apathy.