Little-known fact — well, it’s probably a fact: “Guy” meaning “fellow” (these days, sometimes even including gals), stems from Guy Fawkes, of English Gunpowder Plot (Nov. 5, 1605) fame:
1. Guy Fawkes produced Guy Fawkes Day, which featured burning effigies of him, called “guys.” The Oxford English Dictionary gives, as the first definition of “guy,” “An effigy of Guy Fawkes traditionally burnt on the evening of November the Fifth, [usually] with a display of fireworks. Also in full Guy Fawkes.”
The figure is habited in grotesquely ragged and ill-assorted garments (whence sense 2), and was formerly accompanied by other similar effigies (representing unpopular persons), to which the name of ‘guys’ is often given by extension.
2. That in turn produced “guy” in the sense of “A person of grotesque appearance, [especially] with reference to dress; a ‘fright.’”
3. And this in turn developed in the U.S. into just “A man, fellow”; the Oxford English Dictionary notes that “The earliest examples may be influenced by sense 2,” which is to say “person of grotesque appearance” meaning, which in turn flows from Guy Fawkes.
The name Guy, it turns out, is the Norman French analog of the Italian name Guido. Apparently Guy Fawkes himself went by Guido Fawkes, back in the day.
So, you guys, you rebels you, that’s where your name comes from. Probably.