From Proto-Indo-European *puH-; compare Sanskrit पूयति (pū́yati, “stinks, rots”), Ancient Greek πῦον (pûon, “discharge from a sore”), πύθω (púthō, “to rot”), Gothic 𐍆𐌿𐌻𐍃 (fuls, “foul”), Old English fūl (“foul”) (whence English foul), from the same root.