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Origin documented
“The penny dropped”
What it means
Sudden understanding after a delay.
Where it comes from
Penny-in-the-slot machines were everywhere in late Victorian Britain: weighing machines, automatic stereoscopes, fortune tellers on every pier. The coin frequently stuck, and nothing happened until it fell.
How sure are we?
Attested from the early twentieth century, and the mechanism is the whole point. The pause between inserting the coin and the machine springing to life is exactly the pause the phrase describes.