rap (one) over the knuckles

1. Literally, to smack someone, especially a child, on the knuckles as a punishment for misbehavior. The teacher was fired for rapping students over the knuckles when they spoke out of turn.

2. By extension, to give someone a minor warning, scolding, or punishment. The public was outraged that the government merely rapped the company over the knuckles for illegally dumping chemicals in the river. The boss rapped my knuckles for coming in late on Friday.

rap over the knuckles

A quick, minor punishment intended to serve as a warning. The public was outraged that the company only received a rap over the knuckles from the regulatory agency after being caught illegally dumping chemicals in the river.