filthy
褻 centers on being dirty or impure, especially in a physical or moral sense. The single given meaning 'filthy' captures this idea of contamination.
褻 combines the clothing radical 衣 with the phonetic component 埶. The character likely originally referred to dirty or worn-out clothing, extending to the idea of filthiness in general.
Picture a garment (衣) that has been dragged through the dirt, becoming filthy. The inner part 埶 looks like a messy, soiled tangle, reinforcing the idea of something unclean.
For セツ, imagine a set of filthy clothes piled up: set -> セツ, and the dirty garments need washing.