[nhoward14]

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.

[American Quilter]

Thanks very much for that explanation of "jump the shark", nhoward! I never did understand it until now. And thanks for asking the question, StoneWall.

[Clint N. Suhks]

an episode ran about the Fonz jumping a shark with his motorcycle.

I believe it's when the Fonz jumped over a shark while water skiing.