The Pooplosion, as dubbed by Keefe, is the unofficial name of a salve Physic had taken from the Heks' house.
It was described to be kept in a palm-sized jar, and it was said to be the color white. Its consistency is described to be goopy.
It was used in Neverseen on Fitz after his arthropleura injury. It cleared away the black veins on his chest, and from there it only took an hour and fifteen medicines before Fitz was cured (except for the vile tea).