1
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Phrase(s): beat a dead horse
Go to flog a dead horse.
McGrawhill's American Idioms And Phrasal Verbs
2
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Also, flog a dead horse. Try to revive interest in a hopeless issue. For example, Politicians who favor the old single-tax idea are beating a dead horse. From the 1600s on the term dead horse was used figuratively to mean "something of no current value," specifically an advance in pay or other debt that had to be worked ("flogged") off. [Second half of 1800s]
American Heritage Idioms