jump the shark

phrase

: to undergo a significant change for the worse that marks the point at which a period of success ends (as for a TV series)

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Where did jump the shark come from?

When something jumps the shark it undergoes a significant change for the worse and is on a new trajectory of unrecoverable decline. The happy days of its golden age are over.

The origin of the phrase jump the shark is tucked neatly in that previous sentence: it comes from a 1977 episode of the American TV series “Happy Days” (1974–1984) in which the program's most popular character, Fonzie, jumps over a shark while waterskiing in his trademark leather jacket. Some years later that episode came to be widely identified as marking the beginning of the iconic show's decline, and its plot device became a metaphor for similar transformations:

Nearly all TV shows ever produced have jumped the shark eventually. Such is the nature of television's creative conundrum.
— Monica Collins, Boston Herald, 9 Jan. 2000

Most TV series take three seasons to jump the shark, but in the theater it can happen in 20 minutes …
— Bob Verini, Daily Variety, 18 Sept. 2009

But in its headlong embrace of capitalism and corporate tie-ins, “Sex and the City” may have finally jumped the shark.
— Laura Compton, San Francisco Chronicle, 30 May 2010

The phrase is no longer limited to contexts involving entertainment; anything that undergoes a significant change for the worse that marks the start of a period of decline can be said to have "jumped the shark":

Not everyone agrees when Picasso's art jumped the shark.
— Jeffry Cudlin, Washington Post, 27 Feb. 2011

Silicon Valley has “jumped the shark” and lacks innovation, venture capitalist Peter Thiel says.
— Mike Murphy, MarketWatch, 1 Nov. 2018