The male ejaculate left floating in the tub or shower following masturbation, resembling the marine invertebrate. Generally non-poisonous.
"Dude, go ahead and use my shower but I know you haven't been laid for a while. I better not find any fresh-water jellyfish in there later."
"I would never do such a thing, mate. I'm offended you would suggest it... Any baby oil in the medicine cabinet?"