Green’s Dictionary of Slang

jello n.

[Jell-O the brandname of a gelatine dessert]

an older person, a socially unacceptable person.

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

Brandname in slang uses

In compounds

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

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

posting at DenverPost.com 17 Feb. [Internet] Calling Tasha a ‘jello brain’ is juvenile, but I’m sure she can handle it.
posting on forum JustLabradors.com [Internet] ‘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!’.

2. a drug user [jelly n.1 (7)/jelly bean n. (5)].

[US]Eble Campus Sl. Mar. 4: jello-brain – habitual drug user.
[US]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 squad (n.)

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

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

In phrases

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

a nervous person.

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