Missy is the English diminutive of Melissa — from the Greek melissa (honeybee) — or of Mary/Miriam (Hebrew Miryam, "beloved"). Co-wrote songs for Aaliyah, Tweet, and Janet Jackson.
Missy 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 Missy reduce to 4, The Builder. This is a traditional interpretive system, not a factual claim about the name.