drop a dime

Idioms  

1. Inform on or betray someone, as in No one can cheat in this class—someone's bound to drop a dime and tell the teacher. This expression, alluding to the ten-cent coin long used for making a telephone call, originated as underworld slang for phoning the police to inform on a criminal and occasionally is extended to any kind of betrayal. [1960s]

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Example Sentences

They didn’t break the law to make a billion dollars — at least usually not — so you can’t drop a dime on them.

From New York Times

That means you might have to pay full price to play God of War: Ragnarok when it releases, while Game Pass subscribers won’t have to drop a dime extra to play Starfield.

From The Verge

And William L. Rukeyser of Davis, Calif., pointed out that we still use the expression “Drop a dime” on someone.

From Washington Post

It just got a lot easier to drop a dime on them.

From Washington Times

Is it time to drop a dime to the DMV for re-testing?

From Slate