beat the air

• vi. 与幻想之敌作斗争(作无益的斗争；白费力气；徒劳)

例句

用作不及物动词 (vi.)

1. He was really just ploughing the sands.
   他不过是在白费力气。
2. To argue with such a marl is like beating the air.
   跟这号人辩论是白费力气。