Tamsin is an English feminine form of Thomas — from the Aramaic tʾoma, meaning "twin." Used in the West Country of England since the 16th century.
Tamsin is widely used in the UK. Tamsin Greig (born 1966) is the English actress.
Tamsin 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.
In Pythagorean numerology the letters of Tamsin reduce to 4, The Builder. This is a traditional interpretive system, not a factual claim about the name.