The usage of the word "guy" in common context for people whose names you don't know,

originates with Guy Fawkes.

After the establishment of Bonfire Night, kids would often collect money to build the effigy, asking "money for the Guy?"

Then the word was used to refer to any particularly ugly male person; "Ugh, look at the face on that Guy."

Now, it's so ubiquitous, and has lost all gendered connotations, everyone is Guy.

(Yay for murderous religious fanaticism, I guess.)