Just as it sounds. When fecial matter is compounded in between your cheaks. Word created and then made popular by Dave Chappell doing a skit on his show about the side effects of "Ribs", a sleeping aid for black people.
I got mud butt while laying in bed after eating some ribs.
The feeling of moist, itchy, clay-like fecal matter which remains in your anal cavity after sub-par wiping is the result of one having an extremely messy bowel movement and spending four to six hours perfroming physical activity in moderate to extreme heat. The aforementioned condition may or may not result in the dragging of ones bare anus on a coarse surface such as a carpet or snadpaper.
Jesus Christ, I took a massive shit before helping you move and spent my day scratching my mud butt.