The Probert Encyclopaedia of General Information

FITZ

Fitz is the old Norman French word for fils, son; and was used as a prefix in certain surnames, as Fitzgerald (son of Gerald), Fitzherbert (son of Herbert), Fitzmaurice (son of Maurice), Fitzwilliam (son of William), especially in the surnames of the illegitimate sons of kings or princes of the blood, etc as, Fitzroy, Fitzclarence.
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