INFORMAL I keep trying to get Bob to come dancing with us, but it's just flogging a dead
 horse (=wasted effort) because he says he hates dancing.

 INFORMAL There's no point in flogging yourself to death/into the ground like that
 (=working so hard and making yourself so tired).

 INFORMAL That idea has been absolutely flogged to death (=has been repeated so often
 that it is no longer interesting).

 flogging   noun
 The many floggings (=severe beatings with a whip) that the captain gave his men led to
 the ship's mutiny. [C]
 The punishment for breaking the rules is flogging. [U]
 

flog (SELL)  verb [T] INFORMAL
 to sell, esp. quickly or cheaply
 He tried to flog his old car, but no one would buy it.
 We're flogging our neighbours our lawnmower/flogging our lawnmower to our
 neighbours. [+ two objects]