Myfanwy is from the Welsh my (my) + manwy / banwy (beloved, fine one) — my beloved. Myfanwy is one of the Welsh feminine names — central to traditional Welsh-language heritage. The poem Myfanwy yng Nghaerfyrddin (Myfanwy in Carmarthen) by Hywel ap Einion Lygliw (14th c.) is one of the medieval Welsh love poems.
Myfanwy 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 Myfanwy reduce to 8, The Visionary. This is a traditional interpretive system, not a factual claim about the name.