Green’s Dictionary of Slang

jello n.

[Jell-O the brandname of a gelatine dessert]

[1960s+] an older person, a socially unacceptable person.

Brandname in slang uses

In compounds

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

1. [1960s+] a foolish, scatterbrained person; occas. of an animal.

2. [1980s+] a drug user [jelly n.1 (7)/jelly bean n. (5)].

jello-brained (adj.)

[1960s+] stupid, foolish.

jello squad (n.)

[1970s+] (US campus) an imaginary gathering or club of all those students considered beyond the social pale of their peers on campus.

In phrases

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

[1960s+] a nervous person.