don't beat a dead horse
口語·勸人停止無謂爭論或努力
例句
We've already decided, so don't beat a dead horse by arguing more.
典故來源
源自19世紀賽馬文化,鞭打死馬無法讓它跑起來,比喻對已無結果的事繼續努力是徒勞的。