What is Fitz?

Legal Definition
Fitz (pronounced "fits") is a prefix in patronymic surnames of Norman origin, that is to say originating in the 11th century. The word is a Norman French noun meaning "son of", from Latin filius (son), plus genitive case of the father's forename. Whilst Fitz is now the standard form used by modern historians the word appears in ancient documents with various spellings such as fiz, filz, etc. The word has developed in modern French to fils de, with which it is thus cognate.
-- Wikipedia
Legal Definition
A Norman word, meaning "son." It is used in law and genealogy ; as Fitzher-bert, the son of Herbert; Fitzjames, the son of James; Fitzroy, the son of the king. It was originally applied to illegitimate children.
-- Black's Law Dictionary
Legal Definition
A son.
-- Ballentine's Law Dictionary