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.