'嘴硬' (zuǐ yìng) is an idiomatic adjective meaning 'stubbornly defiant' or 'unwilling to admit defeat or fault,' literally 'hard mouth.' The character 嘴 ('mouth') symbolizes speech or verbal expression, while 硬 ('hard') conveys rigidity and resistance—so together they vividly depict someone who refuses to soften their words, apologize, or concede, even when clearly wrong or under pressure.

This term is commonly used in informal spoken Chinese to describe people who dig in their heels during arguments, deny mistakes despite evidence, or insist on being right for pride's sake. It carries a slightly negative or teasing tone—not outright hostile, but implying immaturity or lack of self-awareness. Parents might say it about children; friends use it playfully among peers; it rarely appears in formal writing. It functions predicatively (e.g., '他嘴硬' — 'He’s being stubborn') and often pairs with verbs like '还' (still), '就是' (just), or '不肯' (refuses to).