Xiu (秀) is the Mandarin Chinese word for "elegant," "refined," or "excellent" — a single character used as a given name on its own or combined with others (Xiulan = "elegant orchid").
Xiu is widely used in China as a feminine name.
Xiu does not currently appear in the US Social Security Administration's top 1,000 girls' names, so we don't publish a US rank or birth count for it. That says nothing about the name's standing elsewhere in the world — only that it sits outside the ranked US data we rely on.
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In Pythagorean numerology the letters of Xiu reduce to 9, The Giver. This is a traditional interpretive system, not a factual claim about the name.