What is the origin of the idiom "rap someone on the knuckles" and when to use it?

The idiom "rap someone on the knuckles" has its origins in the physical act of disciplining students in educational settings. In the past, it was common for teachers or schoolmasters to use a ruler or similar object to lightly strike or "rap" a student's knuckles as a form of punishment for misbehavior or mistakes. This physical action was intended to serve as a deterrent and a reminder to the student to correct their behavior. It is commonly used in situations where a person has made a small mistake, acted inappropriately, or deviated from expected norms.