beat the air 白费力气;徒劳无功;徒劳;白费力

【例句】
His speech merely beat the air as his argument was not convincing at all.
他的演说一点用也没有，因为他的观点不令人信服。

My sister is very stubborn, if you want to change her mind is like beat the air.
妹妹很固执的，你要改变她的心意根本就是白费力气。