blow at a cold coal

• 白费力气；做徒劳无益的事

blow at a cold coal的用法和样例：

例句

1. To argue with such a marl is like beating the air.
   跟这号人辩论是白费力气。
2. It is of no use boasting of being rich on a desert island.
   在无人岛上夸耀富有也是白费力气。

词汇搭配

• a cold coal to blow at 无成功希望的工作...