Peter quotes a proverb about apostates: 'The dog returns to its own vomit, and the sow is washed only to wallow in the mud' (2 Peter 2:22). The term refers to the muddy place where pigs roll and wallow. The image is deliberately repulsive—someone who experiences cleansing but returns to filth is like a bathed pig heading straight back to the mud pit. The proverb illustrates the tragedy of apostasy—knowing the truth but choosing defilement. It's nature asserting itself: pigs will be pigs.