rap across the knuckles
A quick, minor punishment that serves as a warning. The public was outraged that the company only received a rap across the knuckles from the EPA after being caught illegally dumping chemicals in the river.
rap someone across the knuckles
and rap someone on the knuckles; rap someone's knucklesto strike someone on the knuckles. As punishment, she rapped him across the knuckles. The teacher rapped the student on the knuckles.