Dictionary.com
Thesaurus.com
Synonyms

dead horse

American  

noun

  1. something that has ceased to be useful or relevant.


idioms

  1. beat / flog a dead horse, to persist in pursuing or trying to revive interest in a project or subject that has lost its usefulness or relevance.

dead horse More Idioms  

Etymology

Origin of dead horse

An Americanism dating back to 1820–30