The story
From the appearance of the galaxy being reminiscent of a jellyfish.
Etymology from Wiktionary, CC BY-SA 4.0.
Today it means: a type of galaxy created by ram pressure stripping the interstellar gas from a galaxy by the high temperature higher pressure intracluster medium, forming long tendrils of concentrated gas trailing behind the galaxy, which form nebulae and new stars, giving the appearance of celestial jellyfish Full definition →
From the appearance of the galaxy being reminiscent of a jellyfish.
Etymology from Wiktionary, CC BY-SA 4.0.