Lindsay is from the Scottish surname / Old English Lindesege — lind ("linden tree, lime tree") + eg ("island, wetland"). Lindsey Graham is male. The name Lindsay/Lindsey peaked in the early 1980s.
Lindsay 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 Lindsay reduce to 3, The Communicator. This is a traditional interpretive system, not a factual claim about the name.