When you take a poop and it kinda burns your butthole.
Dang guy that crap gave me fire butt.
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."