Arthur, William (1857) An Etymological Dictionary of Family and Christian Names. New York: Sheldon, Blakeman. Public Domain.
SINCLAIR
As described in historical dictionaries
An Etymological Dictionary of Family and Christian Names (1857)
A corruption of St. Clair, and that from St. Clara, from the Latin clarus, pure, renowned, illustrious.
Patronymica Britannica (1860)
SINCLAIR. A corruption of Saint Clair or Saint Clere.
Lower, Mark A (1860) Patronymica Britannica: a dictionary of the family names of the United Kingdom. London: J.R. Smith. Public Domain.
Origin: Celtic
Region of origin: British Isles
Country of origin: Scotland
Religion of origin: Christian
Language of origin: Gaelic