‘反咬一口’ is an idiom that literally means 'to bite back once' — imagining a dog being scolded and suddenly turning to bite its owner. Figuratively, it describes the act of retaliating against one’s accuser by launching a counter-accusation, often unexpectedly or unfairly. The character 反 (fǎn) conveys reversal or opposition; 咬 (yǎo) means 'to bite', evoking aggression and blame; 一 (yì) and 口 (kǒu) together mean 'once' and 'mouth', forming the fixed phrase 'bite once' — emphasizing a single, sharp verbal attack.

This expression is commonly used in contexts involving conflict, disputes, or power struggles — such as legal arguments, workplace disagreements, or interpersonal quarrels — where someone shifts blame onto the person who first criticized or accused them. It carries a strong negative connotation, suggesting defensiveness, dishonesty, or manipulation rather than genuine self-defense. Though rooted in animal imagery, it’s purely metaphorical in modern usage and never refers to actual biting.