It is the Dark butter-like substance that accumulates on the taint and in the asscrack when one has not showered in a long time. It is a combination of sweat, some fecal matter perhaps, and lord only knows what else. It smells like the death of joy, and is the leading cause of swamp-ass. It is disgusting.
Rob: Did you hear that Joe has Black Butter?
Aric: Ew, he better deal with that before it turns into Swamp ass