Ass fu is the fight between the smell of your feces and the smell of the feces of the person next to you in the bathroom. This also works for farts.
I was taking a dump at work the other day and a guy walked into the stall next to me. After a minute, I had to get up and leave because his ass fu was stronger than mine.
Bob cleared out the entire office the other day with his fart. His ass fu is strong.