Basically as the word would imply, someone who has uncontrollable bodily functions in the area of flatulence and poopy stink. Someone who at any given time is known to release copious amounts of both stink farts and smelly poop, or both at the same time.
Farty McPoopbottom's usually tend to be somewhat overweight, but there are rare occasions when a normal size person may become a Farty McPoopbottom.
Guy 1: Man that movie was terrible.
Guy 2: Yeh seriously.
Guy 3: Hey did either one of you guys go in the bathroom at all? Smelled like Farty McPoopbottom in there.
Guy 2: Yeh it made me wana throw up it was so bad.
Guy 1: I didnt really think it smelled that bad.
Guy 3: Yeh well look at what your mom does to your home bathroom on a daily basis, no wonder it didnt smell that bad to you.
*Guy 1 punches Guy 3 in the face*