The penny dropped is a British English, New Zealand English and Australian English idiom that is largely unknown in the United States. We’ll look at the meaning of the idiom the penny dropped, where the term came from and its use in some example sentences.
The penny dropped is an idiom that describes the instant when someone has an epiphany, the instant when confusion clears and a person understands something. Though the term remains in use, the type of situation that the idiom was derived from is seldom seen anymore. The idiom the penny dropped is a figurative term taken from the act of a penny dropping in a slot machine, the metaphor being that nothing happened until the penny dropped into the proper compartment in the slot machine. The slot machines referenced by the idiom the penny dropped are toys or machines that dispensed various goods, not gambling slot machines. The idiom is first seen in the late 1930s. A related term is the penny drops.
When the penny dropped as to the identity of the mystery messenger, she talked to her brothers, who told her to file a complaint. (The Daily Mail)
It was only as I was describing Waterloo Bridge and that photo to my friends that the penny dropped. (The Daily Express)
It took the Goodison crowd a second or two to work out what had happened, but once the penny dropped Naismith’s reward was a generous round of applause. (The Guardian)
The newcomer looked booked for a minor role at best until the penny dropped and she came with a wet sail to deny the running-on Hansian Prince by a neck, although she had to survive a stewards’ inquiry. (The Belfast Telegraph)