The Collaborative International Dictionary of English v.0.48:

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.
   [1913 Webster]