U Jelly is a phrase/meme used on forums, most commonly
4chan, to ask a person if they are jealous. It's usually accompanied by a picture of a guy with a smug smirk, with 'U Jelly?" Over his
head.
It's used to ask forum goers are they jealous, because someone on the forum has something that others don't have, and they want to show it off, to make other
people Jelly.