Green’s Dictionary of Slang

1. (US) common, ordinary.

1901		Monroe & Northup ‘College Words and Phrases’ in DN II:iii 142: jelly-fish, n. as adj. ‘A jelly-fish girl,’ an ordinary sort of girl.
1915		M.G. Hayden ‘Terms Of Disparagement’ in DN IV:iii 218: jelly-fish, very common. ‘Maud is only a jellyfish girl.’.

2. weak, cowardly [i.e. spineless].

1892		Hartford Herald (KY) 27 Jan. 1/4: Jelly-fish Christianity is the greatest danger of these times [...] There are thousands of jelly-fish clergymen and countless jelly-fish sermons [...] Legions of jelly-fish students are turned out of the universities [...] all of which is the result of an unhappy dread of dogma.
1907		W.M. Raine Bucky O’Connor (1910) 101: I’ll leave you and your jelly-fish Scotty to your gabfest.
1942		S. O’Casey Red Roses for Me Act IV: A jully-fush Prostestant!
1963		H.S. Thompson letter 25 Oct. in Proud Highway (1997) 406: I see you got in your dig at that jellyfish bastard.
1990		Guardian 21 Aug. 15/4: ‘Our jellyfish judges and magistrates [...] seem to have far more sympathy with the thug [...] than with the victim’’.