狗嘴吐不出象牙

idiom

a dog's mouth yields no ivory; a mean/foul mouth cannot utter decent words

A metaphorical and slightly humorous proverb used to criticize someone's malicious, vulgar, or ill-intentioned speech. It implies that a person with bad character is inherently incapable of saying anything kind or constructive. The variant '狗嘴长不出象牙 (gǒu zuǐ zhǎng bù chū xiàng yá)' is also common.