The word guy comes from Guy Fawkes, one of the men involved in the Gunpowder Plot of 1605 in England. After the plot failed, people marked November 5 by burning effigies of Fawkes. Those figures were called guys.
From there, the meaning widened. By the 18th and 19th centuries, guy could mean a person dressed oddly or a shabby looking man, probably because the effigies looked ragged. Later, especially in American English, it became a general informal word for man.
Today, the word is even broader in casual speech. Many speakers use guy for one man, guys for several men, and you guys for a mixed group.
- Original sense: They carried a guy through the streets on November 5.
- Later sense: He looked like a guy in old torn clothes.
- Modern sense: That guy is my neighbor.
- Group use: Are you guys ready?
So a personal name became a holiday figure, then a description, and finally one of the most common informal words in English.