The disgusting film of dirt or grease that bums leave behind after sleeping on train seats
Don't sit there, that dirty hobo left behind some bum grease!
by BbStarD June 18, 2011
The product of swamp ass that collects at the top of your butt crack.
My terrible case of swamp ass left a bum grease stain on my underwear.
by Amber Adamson January 24, 2009