In Britain in the 1600s, the word ‘guy’ had a completely different meaning:
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”.
Eventually, the word lost a bit of sting. These days, ‘guy’ is now an unoffensive, informal noun.
Other words that have lost some sting over the years: Samaritan, Bastard