beat a dead horse
US /biːt ə dɛd hɔrs/
UK /biːt ə dɛd hɔrs/
1.
白費力氣, 徒勞無功
to waste effort on something that is no longer possible or cannot be changed
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Trying to convince him now is just beating a dead horse; he's made up his mind.
現在試圖說服他只是白費力氣；他已經下定決心了。
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Let's move on to a new topic; we're just beating a dead horse with this argument.
我們換個新話題吧；我們只是在為這個爭論白費力氣。