"嘴硬" (zuǐ yìng) literally means a tough mouth. It is used to describe a person who knows his or her own faults, but is reluctant to admit.
1. tā zuǐ yìng de hěn, yì shēng dōu bú huì chéng rèn tā cuò le.
He is so stubborn that he has never admitted his own mistakes all his life.
2. Wǒmen yǐ jīng zhǎng wò le zú gòu de zhèng jù, tā zuǐ yìng yě méi yǒu yòng.
It's useless for him to refuse to admit his guilt, because we've had enough evidence on hands.
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