When two people simultaneously take a shit in the wilderness while interlocking arms directly in front of them. Since there are two shits (number two's) taking place at the same time, the sum of the shits becomes a full on Foursie (2+2=4). This task has only been accomplished by two men. No known women have attempted this feat.
The first known Foursies was done on historic ground near Bass lake in California on 7/28/12. The men entered the woods, dropped pants, squatted down, locked hands in front of them, and simultaneously evacuated their bowels. They then ran from a mountain lion that turned out to be a dog.