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