sent coals to newcastle

• v. 做徒劳无益的事(多此一举；白费力气；画蛇添足)

sent coals to newcastle的用法和样例：

例句

用作动词 (v.)

1. To argue with such a marl is like beating the air.
   跟这号人辩论是白费力气。
2. He declared it was of no use to work on his farm.
   他对人家说，在自己的田里干活是白费力气。