She dresses beautifully — but really, a woman adorns herself for one who appreciates her.
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From 《史记·刺客列传》, spoken by the assassin Yu Rang (豫让) when explaining his loyalty to Zhi Bo: 士为知己者死,女为悦己者容 — 'A knight dies for one who truly knows him; a woman makes herself beautiful for one who delights in her.' A paired proverb about reciprocal recognition as the ground of loyalty and self-presentation.
usage
Always cited with the leading half 士为知己者死. The modern reading sometimes flips it into a line about female self-expression for the sake of a lover — but classically it's about mutual recognition, not just romance. 容 here is a verb = 'adorn oneself.'