The person gently smacks the head or forehead.

Realization The person notices a mistake or realizes that he/she forgot something.
“Oh no, what have I done.”

Explanation
The gesture often occurs with honest people. The slight blow to the head is like a symbolic self-punishment. The blow shall re-establish order. A blow to the back of the head or very vigorous rubbing shows that the incident bothers the person.

Categories face

This is one out of 63 gestures from the body language trainer.