beat a dead horse

Example: “We've already decided; stop beating a dead horse.”

Origin: The British form is "flog a dead horse," and it is often credited to the politician John Bright, who around 1867 said that trying to rouse Parliament over a reform bill was like flogging a dead horse — an attribution repeated widely, though the phrase appears to have been in circulation slightly earlier. There is also a nautical "dead horse" with a documented life of its own: sailors commonly received an advance of wages before sailing, and the stretch of the voyage spent working it off was called working the dead horse, marked in some ships by a ceremony in which a straw effigy of a horse was hoisted and thrown overboard once the debt was cleared. Whether the idiom descends from that custom or simply from the obvious futility of striking an animal past responding is not settled.