Green’s Dictionary of Slang

jellyfish n.

[the invertebrate SE jellyfish]

a weak, ineffectual, cowardly person.

[Aus]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.
[US]A.H. Lewis Boss 234: He was a mere jelly-fish of principle — one whose boneless convictions couldn’t stand on their own legs.
[US]Day Book (Chicago) 21 May 17/2: You called me a bone-headed jellyfish.
[US]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!
[UK]Wodehouse Right Ho, Jeeves 196: A shrinking, non-goose-bo-ing jellyfish like Gussie.
[US]W.R. Burnett Little Men, Big World 165: You and the Paymaster! Jellyfish — the only thing holding you up was the Mover.
[US](con. 1960s) R. Price Wanderers 142: Raymond’s a fuckin’ jellyfish. I don’t care how much dough he got.
[US]A. Maupin Tales of the City (1984) 218: You’re a jellyfish, Fielding.
[US]H. Rawson Dict. of Invective (1991) 13: Such specifics as coon, crab, hog, jackass, jellyfish, rattlesnake, skunk, wolf, and worm.