Beat a dead horse：徒劳无益

To beat/flog a dead horse

Definition: To waste time trying to do something that will not succeed.

【世博精解】

如果你去鞭打一头死马，它会有什么反应呢？当然是什么作用也不起。那么这个惯用语就是徒劳无益、白费功夫的意思。英英主要使用Flog a dead horse，而beat a dead horse及flog a dead horse都可见于美英。而且这个短语常常用进行时。如：You’re beating/flogging a dead horse trying to persuade Simon to come to Spain with us-he hates going abroad. 再如：Do you think it’s worth sending my manuscript to other publishers or am I just beating a dead horse?

Example dialog:

A: Let's not talk about it any more. Okay?

我们别谈这件事了，行吗？

B: You're right. There's no use beating a dead horse

你说的对。再谈也是白谈。