Green’s Dictionary of Slang

jelly n.2

also geli’, gellie, gelly
[abbr./ pron.]

1. [1930s+] (orig. Aus.) gelignite.

2. attrib. use of sense 1.

3. [1970s+] napalm.

In compounds

jelly baby (n.)

[1940s–70s] a robber who specializes in gelignite to open safes.