it appears that, once again, Mrs. B. has fallen victim to [benefit of doubt] space limitations:

[AF. spelling of OF. fiz (pronounced fits):—earlier filz:—Lat. filius son.
The form is due to the phonetic law in OF. that a palatalized l caused a succeeding s to become ts (written z).]
The Anglo-French word for ‘son’; chiefly Hist. in patronymic designations, in which it was followed by the name of a parent in the uninflected genitive. Some of these survive as surnames, e.g. Fitzherbert, Fitzwilliam, etc.; in later times new surnames of the kind have been given to the illegitimate children of royal princes. †Also in 12–15th c. used occas. in adopted AF. phrases, beau fitz = ‘fair son’; fiz a putain = ‘whoreson’.