Nga is from the Vietnamese Nga. Nga in Vietnamese tradition refers to the Chinese moon goddess Hằng Nga (Chang'e) — central to East Asian Mid-Autumn Festival mythology. The name appears across Vietnamese feminine naming alongside Hằng Nga, Tuyết Nga, and Thúy Nga.
Nga 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 Nga reduce to 4, The Builder. This is a traditional interpretive system, not a factual claim about the name.