Toilet Crash!
The Grey Prude says it all started in the time before the Larger Pruniverse was created, in a place called Pru's Excrement. "Excrement" is a scientific-sounding noun meaning "shit." It's especially used to describe anything reminiscent of intestinal evacuations. Since these Grey Prude thought balls are a kind of verbal diarrhea, it's the closest word I could find to relay the idea of Pru Poo: something you've inadvertently stepped in, something you desperately want to wipe off the bottom of your shoe.
The Pru-Excrement thought balls are the most difficult to interpret, and they are nearly impossible to describe in polite company. Imagine a long segment of intestines, so long you can't see the oral cavity at one end or the rectum at the other. Imagine looking down toward the sphincter end of the intestines. It's not taut; it's a little loose. Imagine it opening, ever so slightly, to release a barely perceptible stream of fecal matter. Each time the intestines oscillate, a Wave of Diarrhea is blasted out your rectum and into the toilet bowl. You feel like your rectum is raw and exposed; you feel like you had contact with something dirty and disgusting. And then the next Wave of Diarrhea hits you. Infinitely different than the first, it barely registers before the next arrival... and then the next ... and the next...
What strikes you the most is that these Waves of Diarrhea look like the main course you ate for dinner earlier at that high-priced restaurant...
Somehow you know you were served leftovers. Edible garbage. Beautifully arranged on the plate, complex flavors, but somehow flawed, somehow missing something essential, somehow imperfect, somehow covered with bacteria. An assembly line of mostly Prutrid food, this intestinal transport system keeps churning the garbage out. Once in a while a wholly absolute and flawless Wave of Diarrhea strikes, too fast to make it to the toilet on time. You try to focus on your backside, to bring it to the front of your mind, to freeze it in place, but it's long gone. The Pru Poo can't be contained by your mind.
And somewhere early in the thought ball, in the same breath as "cosmic garbage," it knocks you in the gut: one of the ejected Waves of Diarrhea is the Larger Pruniverse.