Green’s Dictionary of Slang

jell n.¹

(Aus.) a coward.

1965		W. Dick Bunch of Ratbags 182: The word jell was a word that was officially made up by our particular group of bodgies in the Goodway Gang. It had been coined about eighteen months ago by us and had since spread throughout the whole of the gang as an accepted bodgie slang-word. Other gangs had also begun to come in contact with the word from us, and were using it as well. The word itself meant coward. To be a coward was to be called yellow. To be yellow meant that the coward was generally shaking like a jelly. We used at first the word jelly, but we shortened it to jell.
1984		Partridge DSUE (8th edn) 615/1: since ca. 1950.

In phrases

jell (it) (v.)

(Aus.) to act in a cowardly way.

1965

W. Dick Bunch of Ratbags 182: I was a bit worried that he might jell it at first and dob us in.