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PP - // … what EVER do you mean by 'jumping the shark'?" //
The phrase 'jumping the shark' refers to one of the last episodes of iconic US comedy series Happy Days when almost all the original stars had left, and the writing was becoming ludicrous. In the episode, Fonzie goes surfing and actually jumps over a shark on his surf board, seen as the nadir of a once great series.
From then on the term 'jumping the shark' means that a series has deteriorated beyond its own parody, and is utterly worthless and to be avoided.