Filthy comes from Middle English filthi, from Old English fȳlþ ('foulness, filth') + -ig; fȳlþ is from fūl ('foul'), from Proto-Germanic *fūlaz, from Proto-Indo-European *puH- ('to rot, stink'). Morphologically, the account is both compositional and suffixal: -ig marks the visible structure by which the older base becomes the modern form. In present use, it is used as an adjective meaning "disgustingly dirty, or morally obscene", so the historical material remains connected with the current sense rather than standing as bare ancestry. The wider English family includes derivatives such as filthily and filthiness and related forms such as filth and foul, showing how the same historical material continues to organize neighboring vocabulary. The named historical forms include Old English fȳlþ and Proto-Indo-European *puH-, keeping the history tied to concrete forms rather than a loose resemblance.