It's when your ass and taint area get all hot and sweaty after doing strenuous activities such as working out or running, thus resulting in the sweat and stinky goo that builds up in your ass crack and taint.
I had a severe case of goo butt after working outside in jeans in 100 degree weather all day.
a fag on call of duty modern warfare 2 and black ops who use the thumper (mw2) or the china lake (black ops)
Guy pulls out thumper - "alright gotta line it up just right, aims in the sky....FOOOMP!!
sweatybawlz - runnin around pwnin noobs...BOOM!!
*watches kill cam*
sweatybawlz- "WOW U FUCKIN THUMPER HUMPER!"