This rare noun means vomit or matter that has been expelled from the stomach. Second Peter 2:22 applies the proverb 'The dog returns to his own vomit' to false teachers who abandon the faith. The term is graphic and repulsive, emphasizing the disgusting nature of apostasy. Peter uses this visceral image to show how returning to former sinful ways after knowing Christ is as revolting as a dog eating its own vomit. The metaphor stresses both pollution and willful degradation.