An Etymological Dictionary of the German Language/faul
faul, adj., ‘rotten, worthless, lazy,’ from the equiv. MidHG. and MidLG. vûl, OHG. fûl; comp. Du. vuil, AS. fûl, E. foul, OIc. fúll, Goth. fûls, ‘decayed’; la- is derivative; fû- as the Teut. root is deduced from OIc. fúenn, ‘putrefied,’ which as a partic. points to an obsolete verb (Goth. *fauan, formed like bauan), of which OIc. feyja, ‘to allow to putrefy,’ is the factitive (Goth. *faujan). From fū̆ several Teut. dialects have formed nouns with the meaning ‘cunnus’ (OIc. fuþ); see Hundsfott. The root fū̆, from Aryan pū̆, is equally represented in the allied languages; Gr. πύον, ‘matter,’ and the equiv. Lat. pûs, n.; Sans. and Zend root pû (pûy), ‘to stink, putrefy,’ Lith. pûvù, půti, ‘to putrefy’ (akin to Lith. púlei, ‘matter,’ with a derivative l as in faul); also Gr. πύθω, ‘to cause to rot,’ Lat. pûteo, ‘to stink,’ pŭter, ‘putrid, rotten.’ The primary meaning of the root pŭ is ‘to emit a smell of putrefaction.’ —