Refers to using shit as lube for anal sex. Typically the receiver with eat food which causes as mushy dump (that kind that takes nine rolls of toilet paper to wipe clean). Rather than wiping, the receiver will leave the remaining shit as is and have the giver insert their penis, using the shit as lube. In some cases, the receiver will cut the shit short, increasing the remaining fecal matter / lube.
We were all out of lube last night, so Sonia just took a dump without wiping and then I fucked her up the ass using nature's lube.