The word "guy" was basically the "f*ckface" of 1600s Britain.
Long story short, Guy Fawkes was a terrorist who tried to blow up Parliament in the infamous Gunpowder Plot of November 5, 1605. He was caught and executed by hanging, something which British people have been celebrating for the past 400 years by building effigies of Guy Fawkes and setting them on fire every November 5th (called Guy Fawkes Night). These effigies, or "guys", were built with wood and old clothing and newspapers before being paraded around and set aflame. Any effigy built for burning, whether representing Guy Fawkes or another person, would from then on be referred to as a "guy".
|A guy, c. 1700|
By the 19th century, "guy" had evolved into a common insult against any man dressed in odd or scrappy clothes.
|A guy, c. 1850|
Eventually it lost its negative connotation and was used to refer to any male person. Today, it can be used to refer to pretty much anyone and is often a term of endearment used among friends.
|A guy, 2013|