Lower, Mark A (1860) Patronymica Britannica: a dictionary of the family names of the United Kingdom. London: J.R. Smith. Public Domain.
We don't have any stats on how common this name is. This is probably because it's very rare in the UK.
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The Forty surname in historical dictionaries
Patronymica Britannica (1860)
FORTY. FORTYMAX. Forty is used by the Scot, poet Douglas, in the sense of brave. Fr. foH. Hence these names probably refer to the courage of their original owners,