The story
- Proto-West Germanic · inherited from*fūliþu
- Old English · inherited fromfȳlþu
- Middle English · inherited fromfilth
- English todayfilth
From Middle English filth, from Old English fȳlþu, from Proto-West Germanic *fūliþu, equivalent to foul + -th (abstract nominal suffix).
Etymology from Wiktionary, CC BY-SA 4.0.