beat a dead horse, flog a dead horsev exprverbal expression: Phrase with special meaning functioning as verb--for example, "put their heads together," "come to an end."
figurative (pursue a hopeless cause)
徒劳无益 tú láo wú yì
白费口舌 bái fèi kǒu shé
I tried to convince him to come with us, but felt that I was beating a dead horse.