When you are so sick (from food poisoning or whatever) that you have to puke and poop at the same time. In fact, you are so sick that you puke into the tank of the toilet, and poop into the bowl at the same time.
Picture an ancient Egyptian painting and how they stand. One hand pointed out front and one pointed back.
Your are puking so bad, and all of a sudden you feel some poop coming out the other side. What ya gonna do? The Egyptian. Get up and straddle the tank. Puke into the tank and poop in the bowl at the same time! That's the Egyptian