The act or art of ruining someone's moment of glory, self-confidence or happiness, usually by putting them down, or showing your superiority over them.
Jim: "I finished in 1:47, bitch."
Bob: "Dude, way to piss on my fireworks..."
John: "Yes! I just got to level 4!"
Pete: "Hah! I've finished the game, arseface!"
John: "Ouch, why do you always have to piss on my fireworks..."