Gamerdeals.net writes:
"'Jumping the Shark':
The phrase refers to a scene in a three-part episode of the American TV series, Happy Days, first broadcast on September 20, 1977. In the third of the three parts of the "Hollywood" episode, Fonzie (Henry Winkler), wearing swim trunks and his trademark leather jacket, jumps over a penned-in shark while water skiing.
Jumping the shark is a colloquialism used by TV critics and fans to denote that point in a TV show or movie series' history where the plot veers off into absurd story lines or out-of-the-ordinary characterizations. In the process of undergoing these changes, the TV or movie series loses its original appeal. Shows that have "jumped the shark" are typically deemed to have passed their peak.
The phrase has been used more recently outside the realm of popular culture, representing anything that has reached its peak and has turned mediocre."