The word "guy" was basically the "f***face" 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 newspaper and scraps of old clothing before being paraded around and set aflame. From then on any effigy built for burning, whether representing Guy Fawkes or another person, would be referred to as a "guy."A guy, c. 1700
In the 19th century, "guy" evolved into a common insult against any man dressed in odd or scrappy clothes. Eventually, it lost its negative connotation and could be used to refer to any male person. Today, it can refer to pretty much anyone or be used as a term of endearment among friends. From terrorist to friend in just 400 years!More questions on Etymology: