A term used to describe a moment when something that was once great has reached a point where it will now decline in quality and popularity.
Origin of this phrase comes from a Happy Days episode where the Fonz jumped a shark on waterskis. This point is generally considered the point where Happy Days stopped being funny.
When a person ditches all their friends to go off alone thinking they're the best and then quickly turns into a doushebag.
"Oh, he has J.T.S."
"Ah I see, what a doushebag."
"You guys are holding me back, I'm going to make my own team and fucking own you all"
"Whatever you say JT"