Rap on the knuckles is an idiom that has been in use for some time. We will examine the meaning of the common saying rap on the knuckles, where it came from, and some examples of its idiomatic usage in sentences.
A rap on the knuckles is a mild rebuke, a scolding, a reprimand. The expression a rap on the knuckles is used when the punishment being discussed is not terribly severe. The phrase a rap on the knuckles came into use in the mid-1700s and evokes the imagery of a child receiving a literal rap on the knuckles with a ruler or switch from a teacher as a rebuke for making a mistake in his lessons or engaging in naughty behavior. Today, a rap on the knuckles is a noun phrase that is often used when the speaker believes someone received a lighter punishment than he should have received.
Since we all know by now how UP functions, we don’t expect them to get anything more than a rap on the knuckles. (Times of India)
Usually, errants are let off with an apology or a minor rap on the knuckles. (Daily Pioneer)
The rap on the knuckles of the media and the Government for communalising the Tablighi Jamaat and demonising the Muslims could, and indeed should, have come from the Supreme Court. (National Herald India)