黄花幼女 (huánghuāyòunǚ): young virgin girl

黄花 ('yellow flower') here refers to chrysanthemum, a symbol of chastity and noble character. By the Song dynasty 黄花闺女 (huánghuā guīnǚ) and 黄花幼女 had become standard literary terms for an unmarried girl. The variant 黄花闺女 is far more common in modern reading.

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Distinctly literary/archaic. Modern speakers are far more likely to say 黄花闺女 (huánghuā guīnǚ) — and even that is dated. Used today mostly in costume dramas and classical fiction.