The Gibbs slap comes from the long-running TV series NCIS, where Leroy Jethro Gibbs became known for smacking Tony DiNozzo on the back of the head. That running gag turned into a widely shared reaction format online, especially in meme compilations, short edits and TV moment reposts. People still use the sound because the joke is instantly readable: someone says something foolish, the slap lands, and the scene gets a clean comedic payoff. Its TV origin gives it recognition even outside the original fanbase.