白费口舌

wǒ zhī dào wǒ shì zài bái fèi kǒu shé

我知道我是在白费口舌。

I can see that I'm wasting my breath.

hé tā men jiǎng dào lǐ zhǐ shì bái fèi kǒu shé

和他们讲道理只是白费口舌。

Trying to reason with them was like banging my head against a brick wall.