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An admission of jealousy. "I'm so jealous". An adaptation of the phrase "miso horny", which is a mutation of the phrase "me so horny", which is a poorly translated version of the phrase "I'm so horny". But with jealousy instead of horn.

Miso jelly could also conceivably be a type of food, which facilitates humourous use of the phrase as a pun.
"OMG you got the latest iButtPlug, MISO JELLY"

"What's the matter baby? Can't finish your supersized bowl of miso jelly?"
by iddles July 10, 2012