The Harley-Davidson Fatboy motorcycle is called "Fatboy" because of its distinctive and muscular appearance. The name was inspired by the atomic bomb, specifically the nickname "Fat Man" given to the bomb that was dropped on Nagasaki during World War II. The original designer of the Fatboy, Willie G. Davidson, wanted to give the bike a strong and powerful image, reminiscent of the bomb's impact. The bike's wide and imposing front end, large tires, and overall robust design contribute to its "fat" appearance, thus leading to the name "Fatboy." The name has since become iconic and is strongly associated with the Harley-Davidson brand.