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A free-swimming, gelatinous, often bell-shaped creature with a basic nervous system and the ability to sting prey. (Sometimes confused with or sometimes seen as moon jelly.)

A weakling or someone who lacks strong character.

Apparently, several interesting euphemisms, all of which can be found right here.
"You're such a jellyfish," said my sister after I dropped my books for the third time and protested that they were heavy.
by tomato-greens December 07, 2005
jello-like organism with no heart and no brain.

see <i><b>boy</i></b>,
So what happened with tht guy you were seeing?
- He turned out to be a total jellyfish
by FOC March 14, 2006
A fish made out of jelly
Jellyfish are jellylike
by Brain Villela January 19, 2005
When a song is your favorite song. It's your "jam"
Wow this song is my jelly fish.
by Great catch August 18, 2014