jello, n. — Green’s Dictionary of Slang

jello n.

[Jell-O the brandname of a gelatine dessert]

an older person, a socially unacceptable person.

1992

Eble Campus Sl. Apr. 6: jello – burned-out older person, bum.

jello-brain (n.) [someone whose brain is like jelly] (US campus)

1. a foolish, scatterbrained person; occas. of an animal.

2008		posting at DenverPost.com 17 Feb. 🌐 Calling Tasha a ‘jello brain’ is juvenile, but I’m sure she can handle it.
2008		posting on forum JustLabradors.com 🌐 ‘She’s a Jello-brain. Everyone told me that having a litter would mature her.....BAH! They lied.’ [...] ‘THEY DID NOT LIE – she’s now a mature Jellobrain!’.

1981		Eble Campus Sl. Mar. 4: jello-brain – habitual drug user.
1989		P. Eckert Jocks and Burnouts 3: Another name [...] for Burnouts is ‘Jells,’ shortened from ‘Jello Brains’ and alluding to the degenerative effects of drugs.

jello-brained (adj.)

stupid, foolish.

1986

C. Hiaasen Tourist Season (1987) 235: He’d just been called a Jell-O-brained moron.

jello squad (n.)

(US campus) an imaginary gathering or club of all those students considered beyond the social pale of their peers on campus.

1975

G. Underwood ‘Razorback Sl.’ in AS L:1/2 61: jello squad n Imaginary or fictitious club of all those regarded as socially unacceptable.

bag of jello (n.) [they shake like jelly]

a nervous person.

1968

Current Sl. II:4 1: Bag of jello, n. A nervous person.