First recorded around the time of the norman conquest of 1066.
Mainly used back then in honor of William the conqurer as it also originates from the name (french origin) William.
Gilliam is the correct spelling of the american surname where as there are many other spellings which were obtained by derilict gypsy prostitutes in england and old time ghetto dwellers of barbados in the 1600's.

Gilliam:(pronounced Gil-am) is best defined as the greatest man ever created. The inventor of the french dip,the haitian fruit basket, and the mongolian teepee. All Gilliams are underworld leaders and have more power than your local government. Very intelligent, and very attractive...Gilliam men exceed expectations in all catagories. Gilliam women inherit the same traits.

Famous Gilliams include:Terry Gilliam,Joe Gilliam,William E. Gilliam,and Scott M.Gilliam

When you eat dirt for the first time and leave your family to build a shack in the woods