Refers to a 1970's TV sitcom. It had a main character known for being the Coolest guy on TV (The Fonze). As the series entered one of its last seasons, the writers had this oh-so-cool guy waterskiing in his trademark leather jacket, to jump a shark. It was truely the moment when the series went downhill from there and was soon mercifully cancelled.
Rober Jordans book series 'The Wheel of Time' jumped the shark in book four and has gotten worse ever since.