When you're having anal sex and the person on the receiving end bleeds and shits on you're dick while you cum.
Chris: Dude this one time Sally and I were doing anal and she bled out of her asshole and she fudged my stick dude once I came.
Josh: dude no way she gave you a tuff mudder!!!!!!!!
Someone who shits right after someone else, and can feel the heat on the seat.
My pal Dave had to go so badly he pulled a real tuff mudder at the bar. What a champ.