Siiri is from the Finnish Siiri — Finnish rendition of the Old Norse Sigríðr — Sigr (victory) + fríðr (beautiful). Siiri is one of the Finnish feminine names — consistently in the top 10 Finnish girls' names of the 2010s-2020s.
Siiri 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 Siiri reduce to 1, The Leader. This is a traditional interpretive system, not a factual claim about the name.