Thoreau is a French-origin surname — possibly from the Old French thor (bull, strong). Helen Thoreau — his older sister. The Thoreau name remains rare in American naming but holds powerful literary and philosophical weight. The Thoreau Society is the oldest literary society in the United States dedicated to a single author (founded 1941).
Thoreau 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 Thoreau reduce to 7, The Seeker. This is a traditional interpretive system, not a factual claim about the name.