Not the phrase, the actual episode of Happy Days in which Fonzie jumped a shark. Fred Fox Jr., the writer of the episode, writes:
Contrary to pop culture belief, when Fonzie jumped the shark, it hardly marked the demise of the show.
In 1987, Jon Hein and his roommates at the University of Michigan were drinking beer and had Nick at Nite playing in the background. They started talking about classic TV shows when someone asked, “What was the precise moment you knew it was downhill for your favorite show?” One said it was when Vicki came on board “The Love Boat.” Another thought it was when the Great Gazoo appeared on “The Flintstones.” Sean Connolly offered, “That’s easy: It was when Fonzie jumped the shark.” As Hein later recounted, there was silence in the room: “No explanation necessary, the phrase said it all.”
Thus was born an expression that would quickly make its way into the pop culture mainstream, defined by Hein as “a moment. A defining moment when you know from now on … it’s all downhill … it will never be the same.” If I had been in the room, however, I would have broken that silence of self-assuredness, for I wrote that now infamous episode of “Happy Days.”
And more than three decades later, I still don’t believe that the series “jumped the shark” when Fonzie jumped the shark.
The rest here.