A gnarly dookie, aka a fatty shit, aka a large fecal excretion. Often times it is a result of eating too much food. It can also be the result of having problems such as Irritable Bowel Syndrome or Lactose Intolerance.
Person 1: Dude i just took a huge ass gnardook
Person 2: Use some fuckin' febreeze, it smells like you shat out a rotting gerbel carcus!