Lower, Mark A (1860) Patronymica Britannica: a dictionary of the family names of the United Kingdom. London: J.R. Smith. Public Domain.
GILLIAM
As described in historical dictionaries
Patronymica Britannica (1860)
GILLIAM. See William.
Surname type: From given name or forename
Origin: Celtic
Region of origin: British Isles
Country of origin: Wales
Religion of origin: Christian
Language of origin: Welsh