The worst kind of diarrhea, usually occurring after a bowel-angering meal. Typical stages of the mud include, but are not limited to: farts you don't want to let out for fear that it won't only be air, followed by 20+ minutes of anal hell. Allow 15 minutes for cleanup.
Colin: *knocks on bathroom door* Chris, I need in there! The Chipotle we ate wants out!
Chris: I can't leave....its the mud...THE MUUUUD!!
Colin: My God!