flog a dead horse
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Related to flog a dead horse: Beat a dead horse
flog a dead horseand 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.
flog a dead horse(British, American & Australian) also beat a dead horse (American)
to waste time trying to do something that will not succeed (usually in continuous tenses) You're flogging a dead horse trying to persuade Simon to come to Spain with us - he hates going abroad. Do you think it's worth sending my manuscript to other publishers or I am just beating a dead horse?