The English surname Gilliam is of patronymic origin meaning son of or descendant of Guillaume. The personal name Guillaume is a central French version of William. The name Gilliam was originally brought to England by Normans during the Norman invasion of the 11th century. The name Gilliam is first found in Essex where members of the Gilliam family were granted lands by William the Conqueror in 1066. A noted bearer of the name is the Film director Terry Gilliam.