Fitzwilliam, lit. "Son of William", is derived from the Anglo-Norman prefix Fitz often used in patronymic surnames of Anglo-Norman origin; that is to say originating in the 11th century ; and from William, lit. "Willpower/Desire Protector", which is a popular given name of old Germanic origin, becoming very popular in the English language after the Norman conquest of England in 1066, and remained so throughout the Middle Ages and into the modern era Wikipedia