Flogging a dead horse

"Flogging a dead horse" (British, American & Australian), beating a dead horse (USA), or beating a dead dog (in some parts of the Anglophone world). It was first coined by the English politician and orator John Bright, in reference to his campaigning experiences in relation to the introduction of the Reform Act of 1867, which called for more democratic representation in Parliament.


 * To continue a particular endeavor that is a waste of time as the outcome is already decided.


 * Source: http://idioms.thefreedictionary.com/flog+a+dead+horse and https://en.wikipedia.org/wiki/Flogging_a_dead_horse