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drop a dime

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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The American Heritage® Idioms Dictionary Copyright © 2002, 2001, 1995 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company.