Hillary is from the Latin hilaris ("cheerful, joyful") — same root as "hilarious." Originally a male name (Saint Hilary of Poitiers, 4th c.); used for women from the 1960s. Yale Law-educated lawyer who graduated 1973.
Hillary 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 Hillary reduce to 4, The Builder. This is a traditional interpretive system, not a factual claim about the name.