rap (one's) knuckles
1. Literally, to smack someone, especially a child, on the knuckles as a punishment for misbehavior. The teacher was fired for rapping students' knuckles when they spoke out of turn.
2. By extension, to give someone a warning, verbal scolding, or a minor punishment. The public was outraged that the government merely rapped the company's knuckles for illegally dumping chemicals in the river. The boss rapped my knuckles for coming in late on Friday.
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]