rap someone's knuckles

verb3

1. •
   to tap one's knuckles on an object such as a desk or wall to attract attention or as a signal of commmunication.

   • Just rap your knuckles on the wall when you're ready to go.
2. •
   to give someone a punishment.

   • If the sales department doesn't get organized soon, the CEO is going to rap their knuckles.
3. •
   (no longer used) to strike someone on the back of the hand usually as a method of punishment.

   • In the old days the teacher would rap someone's knuckles with a ruler if they misbehaved in class.