The story
From one percent + -er. After Thomas Edison, who wrote: "Genius is one percent inspiration and ninety-nine percent perspiration." The motorcycling sense comes from a supposed statement made by the American Motorcycle Association in 1948 that 99% of motorcyclists are decent people enjoying a clean sport, and it is only 1% who are antisocial.
Etymology from Wiktionary, CC BY-SA 4.0.