The word or word fragment came from the Norse French and Latin for "son of" and only occasionally denoted bastardy (which is
never the fault of the child, hence I don't use "bastard" as an insult).
Its use during the period of English surname adoption led to its incorporation into patronymic surnames, and at later periods this form was adopted by English kings for the surnames given some of their recognized illegitimate children, and by Irish families when anglicizing their Gaelic patronymic surnames.
several illegitimate children of the Norman and early Angevin kings were called 'fitz Roy', "son of the king" in Anglo-Norman French, examples being Henry fitz Roy, son of Henry I, and Richard fitz Roy, son of king John.
From the Stuart era (16031714) and later, there was a revival of the adoption of Fitz surname forms, particularly for illegitimate children of kings, princes, or high nobility, for example Fitzroy for the children of Charles II and one of his mistresses, the Duchess of Cleveland; FitzJames, for the illegitimate children of king James II (16851688) and Arabella Churchill; FitzClarence for those of Duke of Clarence, later King William IV (18301837) by Mrs. Jordan; and FitzGeorge, for the sons born to the legally prohibited marriage of Prince George, Duke of Cambridge (18191904) with Sarah Fairbrother, who would refer to herself as Mrs. FitzGeorge. This practice by the late royalty gave rise to the erroneous belief that historical instances of Fitz surnames also denoted illegitimacy, which was not the case.[3]
https://en.wikipedia.org/wiki/Fitz