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rap someone's knuckles

Reprimand, as in If I'd seen John take that last piece of cake, I'd have rapped his knuckles. This term transfers a physical punishment to a verbal one. [Late 1600s]

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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.