Fitz \Fitz\, n. [OF. fils, filz, fiz, son, F. fils, L. filius. See Filial.]

A son; -- used in compound names, to indicate paternity, esp. of the illegitimate sons of kings and princes of the blood; as, Fitzroy, the son of the king; Fitzclarence, the son of the duke of Clarence.

Source: Webster's Revised Unabridged Dictionary (1913)