Green’s Dictionary of Slang

a weak, ineffectual, cowardly person.

1902		Bulletin (Sydney) 25 Oct. 15/4: As a matter of fact, Schriener is the jellyfish of S.A. public life. He is an Imperialist of the most accommodating order. [...] Had he shown backbone, Chamberlain almost certainly must have heeded him.
1903		A.H. Lewis Boss 234: He was a mere jelly-fish of principle — one whose boneless convictions couldn’t stand on their own legs.
1915		Day Book (Chicago) 21 May 17/2: You called me a bone-headed jellyfish.
1923		H.C. Witwer Fighting Blood 119: He loathes the rest of the fellows in our crowd, says they’re a lot of spineless young jellyfish!
1934		Wodehouse Right Ho, Jeeves 196: A shrinking, non-goose-bo-ing jellyfish like Gussie.
1952		W.R. Burnett Little Men, Big World 165: You and the Paymaster! Jellyfish — the only thing holding you up was the Mover.
1974		(con. 1960s) R. Price Wanderers 142: Raymond’s a fuckin’ jellyfish. I don’t care how much dough he got.
1978		A. Maupin Tales of the City (1984) 218: You’re a jellyfish, Fielding.
1985		B. Robinson Aussie Bull 33: She told me that it should be an object lesson to me not to be a ‘spineless jellyfish’ and in future to ‘stand up for my rights!’ .
1989		H. Rawson Dict. of Invective (1991) 13: Such specifics as coon, crab, hog, jackass, jellyfish, rattlesnake, skunk, wolf, and worm.
2001		D. Looser Boobslang [U. Canterbury D.Phil. thesis] 95/2: jellyfish n. an inmate unable to handle the pressures of prison life.