See here.
What is jump the shark?
It comes from "Happy Days" when an episode ran about the Fonz jumping a shark with his motorcycle.
"jump the shark" per Wikipedia...
Jumping the shark is a metaphor that was originally used to denote the tipping point at which a TV series is deemed to have passed its peak, or has introduced plot twists that are illogical in terms of everything that has preceded them. Once a show has "jumped the shark", fans sense a noticeable decline in quality or feel the show has undergone too many changes to retain its original charm. The term has also evolved to describe other areas of pop culture, including movie series, music, or acting celebrities or authors for whom a drastic change was seen as the beginning of the end. These changes are often attempts to attract their fans' waning attention with over-the-top statements or increasingly overt appeals to sex or violence. Some have broadened its use to simply describe any decline in appeal for the subject in question, without requiring a significant "jump the shark" moment as justification.
Jumping the shark is a metaphor used by television critics since the 1990s. The phrase, popularized by Jon Hein on his web site www.jumptheshark.com, is used to describe the moment when a television show or similar episodic medium is in retrospect judged to have passed its "peak" and shows a noticeable decline in quality. Hein also uses the "jumping the shark" concept to describe other areas of pop culture, such as music and celebrities, for whom a drastic change was the beginning of the end.
en.wikipedia.org/wiki/Jump_the_shark