https://en.wikipedia.org/wiki/Jumping_the_shark

The phrase was coined in 1985 by radio personality Jon Hein in response to a 1977 episode from the fifth season of the American sitcom "Happy Days", in which the character of "Fonzie"(Henry Winkler) jumps over a live shark while on water-skis.

Basically any time a show goes on too long and tries to introduce a stupid, attention-getting gimmick to try to stay relevant.

In the old sitcom "Happy Days," after far too many seasons, a new episode featured The Fonz, blue screen water skiing, a crappy looking shark prop, and the Fonz literally jumped over the shark.

Its from the show Happy Days (1977) where Fonzie jumps over a shark while water skiing. Its considered the point where the show took a dramatic turn down hill and the term is still used in that manner today.

It's from Happy Days when the Fonz literally jumped over a shark while water-skiing. Seen as a sign that the show is out of ideas and using crazy stunts or out-of-character actvities to shock some life (money) out of a dying franchise.