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general::
Phrase(s): flog a dead horse [and] beat a dead horse
Fig. to insist on talking about something that no one is interested in, or that has already been thoroughly discussed. • The history teacher lectured us every day about the importance of studying history, until we begged him to stop flogging a dead horse. • Jill: I think I’ll write the company president another letter asking him to prohibit smoking. Jane: There’s no use beating a dead horse, Jill; he’s already decided to let people smoke.
McGrawhill's American Idioms And Phrasal Verbs