Green’s Dictionary of Slang

(US teen) an appalling, unacceptable person.

1982		M. Pond Valley Girl’s Guide to Life 59: Jel — OK, like a Jello-brain [...] a total geek.
1989		P. Eckert Jocks and Burnouts 70: When people go into classifying, you know, you got the Jocks and the Jells.
2002		‘Valley Girls’ on Paranoiafanzine 🌐 OK....you’re like, a total jel. I mean, you’re hopeless. So, uh, kiss my tuna, okay.