British slang.

The art of putting a dampener on someones moment of glory/celebration.

Interchangeable with the phrase "You rained on my parade."
Person A: I got 3As in my exams! Awesome!
Person B: I got 7As.
Person A: Way to piss on my bonfire, bastard.
by Espiria June 13, 2006
Get the piss on my bonfire mug.