From the Smithsonian:
"The word has surprisingly black roots: It’s derived, etymologists believe, from the name Guy Fawkes, one of the leaders of the Gunpowder Plot that attempted to assassinate King James I in 1605. Years after Fawkes’s plan was foiled, British children paraded his effigy around on Nov. 5 — a custom that, over the course of decades, made “guy” a sort of slang, first for a poorly dressed person, and then more generally for a man ( … of any wardrobe)."
I find the possibility of this being true simply amazing.
Full story: here.