The nasty, scumy, build up of sweat, crusted feces and other various substances in and around one's anus. Usually an indicator of uncleanliness and/or failure to correctly wipe one's rectum after defecation.
"Frank you need to wipe man, I smell that ass nast from all the way over here!"
by B729 November 28, 2007
Get the ass nast mug.