The solipsistic shit (or solipshit) is the shit you know you've taken, but when you check the bowl, there's nothing in there. This actually has a logical explanation. 9 times out of 10 when this occurs, the shit you took was so powerful and so heavy, that you basically dropped an anchor in the bitch, and the shit sunk to the bottom of the toilet half way down the drain and you just didn't see it.
Person 1: Man, I just this massive shit and I know I did, but there was nothing in the toilet when I looked.
Person 2: Looks like you've experienced the solipsistic shit.
This is the shit that you have to hold in for whatever reason, but you can fuckin feel it get more powerful by the second as it builds up and waits for its moment to strike. And you know: any moment could be your last. You're searching for an adequate restroom just to wreak havoc. When you finally find it, you release a diarrhea octopus upon the world.
Holy fuck, I had to hold in this massive harbinger of terror all day. It was a mess when I finally could use the restroom.