Arthur, William (1857) An Etymological Dictionary of Family and Christian Names. New York: Sheldon, Blakeman. Public Domain.
FORSYTHE
As described in historical dictionaries
An Etymological Dictionary of Family and Christian Names (1857)
(origin: Gaelic.) From Fear, a man, and Syth, upright, honest, stiff.
Origin: Celtic
Region of origin: British Isles
Country of origin: Ireland
Religion of origin: Christian
Language of origin: Gaelic