Meaning of Fitz

I turns out that "Fitz" is a corrupton of the Latin filius, meaning "son". It later became used for illegitimate ascriptions of royalty. There seems to be a common misconception (that I was also under) that "Fitz" means "illegitimate child". I'm glad to have had that clarified, b/c a guy at work likes to be called "Fitz" for short. He spells his last name, "FitzGerald" with a capital "G", claiming that that's more authentic than, "Fitzgerald". It kind of makes sense, b/c "McDonald" looks more typical than "Mcdonald".