The final /θ/ of filth no longer plays any useful morphological function. It has become fused with its derivational base into an indivisible whole. This is quite often the terminal stage in the life-cycles of linguistic replicators. Old English -þ- was still a morpheme, but it had already lost most of its phonological substance. A few hundred years earlier, in Proto-Germanic, its ancestral form had been *-iþō, continuing still earlier (pre-Germanic) *-étā. A linguistic entity that used to be a suffix of some length has ended up as phonological raw material. It means nothing by itself and has degenerated into a speech sound which, together with three others, encodes a meaning (or rather a cluster of meanings) but is no different, as far as its status is concerned, from the final /m/ of film.
Whole words may become reduced to the role of ‘bound’ (non-independent) morphological elements. Many derivational affixes used to be words which, through being frequently used in composition, survived in that function while their free-standing variant went extinct. Old English hād meant ‘person, social status’. When added to a noun it meant ‘the state or condition of being an X’. Hence, for example, OE ċild-hād ‘infancy, childhood’. The word hād > hǭd lingered on in Middle English, but seems to have become rare by the thirteenth century and eventually died out as an independent word. Curiously, in modern ‘gangsta’ slang hood (no connection with hood = ‘head covering’) is used as an abbreviation of neighbourhood. It has become a word again, though with a brand new meaning.
|Words have fractal-like properties: the more closely you look at them,|
the more structure they reveal
When such reduction and fusion processes have operated for millennia, they may compact a whole string of morphemes into a short word without any visible internal structure. If you look at young /jʌŋ/ today, it’s short even for an English word. In the reconstructed remote ancestor of English, the Proto-Indo-European language, it looked roughly like this: *h₂ju-h₃n̥-ḱó-s. The first element, *h₂ju-, was the compositional variant of the noun *h₂óju ‘vitality, youthful vigour’; the second was a suffix (possibly derived from an independent word) meaning ‘having, loaded with’. Together they formed the noun *h₂jú-h₃on- meaning ‘energetic young man’ (literally: ‘having the strength of young age’, cf. Skt. yúvan-). The addition of the suffix *-ḱó- produced an adjective with the meaning ‘like a young man, juvenile’. We find its reflexes for example in Sanskrit (yuvaśá-), Latin (iuvencus), Welsh (ieuanc ~ ifanc), and of course in the Germanic languages (PGmc. *jungaz > OE ġeong ~ iung [juŋg] > young). In other words, the /jʌ/ part of young is what has remained of a once independent noun, and the /ŋ/ represents two concatenated morphemes compressed into a single segment. Incidentally, *h₂óju is a very interesting item in the Proto-Indo-European lexicon, and I hope to return to it soon.