It's when after someone takes a shower, they stop the drain and leave a little water left in the tub. Then they ejaculate in the small pool and leave the "jellyfish" there for the next person/roommate to discover.
you live in an apartment, dorm, or house - where a bathroom is shared. the Jellyfish is at the next level of disgusting pranks!
A kind of faux five when you're going in to give someone a high five but quickly pull away, making your hand make jellyfish motions in the air while saying, "JELLYFISH!"
To add to the affect, apologize, then go in for a hug.
When they go in for the hug also, pull back and make the jellyfish motions with your arms while saying, "JELLYFISH!"