Jerrying

The process of rapidly slapping one's forehead with the right or left hand. It is done for entertainment and often for attention from an individual or group. This term was developed by a school student in the US and has since become popularized by students across the country. While its specific origin is unknown, the previous statement is where "Jerrying" is most often attributed.

Derek: "Why is he slapping himself?"
Matt: "Oh, he is just Jerrying!"