The term "one-armed bandit" originates from the early days of slot machines, which were designed with a single lever on the side that players would pull to spin the reels. The "one-armed" part refers to this lever, while "bandit" reflects the perception of these machines as a means of losing money, similar to how a bandit would rob someone. The term conveys both the mechanism of play and the potential for financial loss, capturing the allure and risk associated with gambling.