When you have a poop which is equally as devastating as the Hiroshima atomic bombing. The shit must be pungent to the point that it is lethal for humans to be around and will make the leaves fall off trees. Size is another defining feature--it must be massive.
I just had a Hikoshimi Diarrhea! Everyone get outside!
Ew! Who just Hikoshimi'd in the washroom! That is disgusting