One of the mysteries of the English language finally explained.
Designating or relating to a machine or mechanical device (as a weighing machine, dispensing machine, etc.) actuated by the fall of a penny inserted through a slot or narrow opening.
A penny-in-the-slot machine. rare.
Late 19th century; earliest use found in The Boston Globe. From penny + in + the + slot.
penny-in-the-slot/ˌpɛni ɪn ðə ˈslɒt/
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