Mad Dog Fist (狂拳) is a technique used by Goku. Pretending to be a wild uncontrollable dog, Goku catches his opponent off guard eventually knocking him out with a swift kick by jumping from behind.
Crazy monkey technique
Goku mistakenly calls this technique Kyo-ken in the manga, which Master Roshi (disguised as Jackie Chun), yells at him for ("You've got your Martial Arts words wrong! Kyo-ken means MAD DOG!"). An alternate name for this techinque is the Crazy Monkey Techinque (Saru-ken), as it is called in some versions of the anime.