a sack of crap hanging from ones rectum between their legs as to appear to be ones testicles
"tickle me bum sack"
by Captain BallTouch June 19, 2012
A sack that four or more bums crap in because they have no toilet
paper or plastic and sucks if rips
by Mich2 August 20, 2005
A sack which hangs from the buttocks of millions of North Americans.
"Look at that gnarly bumsack."
by squackerdoodle August 19, 2011