Headslap or Gibbsslap is the term used to describe NCIS Special Agent Leroy Jethro Gibbs slapping the members of his team on the back of the head if they're getting off topic or if they're just acting like idiots in general.
Nearly all of the members of the NCIS Major Case Response Team that Gibbs leads except for Forensic Scientist Abigail Sciuto, Medical Examiner Donald Mallard and deceased Special Agent Caitlin Todd have gained a few head-slaps with DiNozzo being the one who receives the most headslaps, usually for his childish behavior.
It was later revealed in a flashback in Hiatus Part 1 (episode) that Gibbs's former boss and mentor, Michael Franks was the one responsible for starting the head-slapping trend, a trait that Gibbs has kept on going to this day ever since he became the leader of the NCIS Major Case Response Team.
In addition to Gibbs using the headslap, other members of the team including DiNozzo himself and Abby have been seen using the headslap as well. Ducky has head slapped Dinozzo twice so far. Once with a file to get his attention and again after his failed impression of him.
In The Bone Yard (episode), Gibbs remarked that a slap to the face would be embarrassing and as such, a slap to the back of the head serves as a wake-up call.
A flashback in the Season 8 episode, Baltimore (episode) showed that Gibbs had given Tony a headslap the day Tony had arrived at NCIS Headquarters, implying that this was the first ever headslap Tony had received from Gibbs.