one-armed bandit

noun Informal.

1. slot machine(def 1).

noun

1. a fruit machine operated by pulling down a lever at one side

A slot machine, as in It’s amazing how many people think they can make money playing a one-armed bandit. This term refers to both appearance and function: the operating lever looks like an arm, and the machine in effect robs players, since it “wins” and keeps the player’s money in an overwhelming majority of instances. [c. 1930]