The term jumping the shark, I discovered today, is an idiom that is a way of describing when a television show’s quality begins to decline. It originated from the television show Happy Days in its season five premiere, where The Fonz literally jumped over a contained shark on a beach front.
This was quite a step away from its usual family and adolescent related story lines set against the nostalgia of the 1950s.
The usage of “jump the shark” has subsequently broadened beyond television, indicating the moment when a brand, design, or creative effort’s evolution declines.Wikipedia, Jumping the shark
And here’s the clip: https://www.youtube.com/watch?v=t4ZGKI8vpcg