It is the Latin for William, I suspect you are looking at early baptism registers when Latin was used in the PRs. You will no doubt find more Latin than just latinised names. Latin would have been used in the church when England was a Roman Catholic country and this why you will find Latin still in use after the reformation. Regards Jenny DeAngelis Spain, (where William is spelt as Guillermo) Hi ive come across this strange forename Guilielmi / GUILIELMUS .Does anyone know where is originates from ? also what other forenames derive from it. His son had the same name. sue