Lower, Mark A (1860) Patronymica Britannica: a dictionary of the family names of the United Kingdom. London: J.R. Smith. Public Domain.
FORTY
As described 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,