1. One who has so much negativity built up inside that they practically spew negativity/negative like a fountain spews water.
2. One who makes rude comments towards another without being provoked.
3. One whos only method of feeling better is to try making you feel as shitty or shittier than they do.
Leo: srsly, ur dumbbb!!!
Tom: "u are just a fucking negativity fountain these days!l"
Jack: "I'm on vacation all week!"
Jill: "It'll probably rain."
Jack: "well at least i got my paycheck today."
Jill: "but ur car insurance is due and u owe me 40 bucks!"
Jack: "you don't have to be such a negativity fountain!"