Thi is from the Vietnamese Thị (the feminine particle, indicating female lineage) — also from Thi (poetry, derived from Chinese 詩). Notable bearer: Trần Thị Lý — Vietnamese woman whose bravery during the war was memorialized in Tố Hữu's poem Người con gái Việt Nam (1959).
Thi 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 Thi reduce to 1, The Leader. This is a traditional interpretive system, not a factual claim about the name.