flog a dead horse
白费力气，做徒劳无益的事情
口语·形容白费力气
英文释义
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.
典故来源
源自18世纪英国，鞭打（flog）死马（dead horse）毫无意义，比喻做徒劳无功的事。