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1.
Jelly is a slang word that derived from the word "jealous".
It's a play on words: you're so jelly you turn into jam. Basically saying that you're extremely or beyond envious.

Mostly used in a joking manner and among females.
1: I'm so jelly I'm jam, that you're on vacation right now Beth.
by iknowthings100 March 28, 2013