Crazy Fist (狂拳) is a distraction technique used by Goku and designed to confuse and taunt the opponent. Pretending to be a wild uncontrollable dog or monkey, Goku catches his opponent off guard, and eventually knocks him out with a swift kick by jumping from behind.
Goku uses the Crazy Fist in reaction to Jackie Chun's use of the Drunken Fist during the 21st World Martial Arts Tournament. Barking and jumping around to scare Jackie Chun, he then jumps behind and kicks his surprised opponent.
In the Dragon Ball manga, Goku uses the Mad Dog and the Crazy Monkey techniques (猿拳, Saru Ken; lit. "Monkey Fist") one after the other, while in the anime, he only uses the Crazy Monkey technique. Goku mistakenly calls this technique Kyo-ken in the manga, which Jackie Chun yells at him "You've got your Martial Arts words wrong! Kyo-ken means MAD DOG!"