flog a dead horse
白費力氣，做徒勞無益的事情
口語·形容白費力氣
英文釋義
to persistently pursue or continue with a task or argument that is futile or pointless
例句
Stop flogging a dead horse; the deal is already off.
典故來源
源自18世紀英國，鞭打（flog）死馬（dead horse）毫無意義，比喻做徒勞無功的事。