save your breath idiom informal
不值得浪费口舌；不必对牛弹琴
used to say that it is not worth talking to someone because they will not listen to you
I don't know why I even try to talk to him - I might as well save my breath. 我不知道为什么要费口舌和他说话——最好还是不要对牛弹琴。