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The Running Gibbs Joke In NCIS That Never Got Old

So far, the CBS crime procedural "NCIS" has run for 19 seasons and 435 episodes, with Season 20 scheduled to premiere this fall. The show's bread-and-butter is its case of the week, usually a mysterious murder somehow connected to the U.S. Navy. But the show also has a strong cast of memorable characters who are easy to root for.

Traditionally, the show's heart and soul has been Supervisory Special Agent Leroy Jethro Gibbs (Mark Harmon). A gruff ex-Marine of few words, he functioned as the show's "straight man," the foil to the rest of the cast's eccentricities. That's not to say Gibbs is entirely humorless, though. Occasionally, he lets the mask slip and allows his playful side to shine through. (This is one reason why fans were so bummed when Harmon left the show, although it's always possible Gibbs might return).

Here's one recurring Gibbs gag that never outstayed its welcome.

Fans love the patented Gibbs head slap

Yep, fans love it whenever Gibbs smacks one of the members of the team on the back of the head. This YouTube compilation of Gibbs head slaps has over 1.2 million views, and more than a thousand comments.

"Gotta love a classic Gibbs head slap," wrote YouTube user Kayleigh Natasha.

"I swear, if I could meet Mark Harmon in real life I'd pay him to slap the back of my head just for nostalgic laughs," wrote YouTuber Deep.

For YouTuber David Forster, the best all-time Harmon head slap was when Ziva (Cote de Pablo) visited an amnesia-stricken Harmon in the hospital, grabbed his hand, and smacked the back of her own head with it in the hopes that it would jog his memory. (It didn't).

Mark Harmon once explained the origins of the head-slaps. They began when his co-star Michael Weatherly, who played Tony DiNozzo, strayed off script in a scene involving DiNozzo flirting with a woman. Harmon gave him a smack on instinct, and the writers turned it into a running joke. Most of the following head slaps targeted DiNozzo, although Gibbs did give Tim McGee (Sean Murray) plenty of hard knocks as well.