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3.
The incredibly painful stinging sensation one receives on their anus, usually caused by indigestion and/or spicy foods.
I love hot wings, but they give me firebutt.

Sorry honey, we can't do anal tonight, I've got firebutt.
by CritzCrakka August 26, 2006
 
1.
When you take a poop and it kinda burns your butthole.
Dang guy that crap gave me fire butt.
by fatpat March 28, 2006
 
2.
1. the sensation that comes when one eats some spicy food (maybe some korean barbecue or a volcano box from taco bell) and takes a dump later on during the day, and feels as if his or her butt knew that the food was spicy.

2. a feeling burning around the anus after eating spicy foods and eliminating the resulting stool

note: the foods causing fire butt are not limited to spicy food.
"Hey, how's it going"

"Man, I had some volcano nachos this afternoon and I just took a deuce. Feels like my ass is on fire."

"You, my friend, have fire butt."
by kevlarcasm December 15, 2009