Adjectival use seems to be attested in several
place names (Amon Dín "Silent Hill", etc.),
though an adjective dínen.1 is also attested
(Rath Dínen "Silent Street"). When compared with other toponyms where
lenition does occur (Taur-na-Chardhîn "Forest of the Southern Silence"
in WJ/185,193 and Dor Dhínen in WJ/333,338), the forms dîn and
dínen clearly seem to be unmutated. Absence of lenition in these
examples from LotR was therefore tentatively explained by resistance to
mutation (as in Nan Tathren, Ered Mithrin).
However, Tolkien apparently changed his mind in his unfinished
index of names from LotR, where he explains both words as mutated adjectives
whose unlenited forms are respectively tîn.2
and tínen . Such hesitations between mutated and
unmutated forms is not unusual, for instance a similar issue is met
with gaear and aear .
Of course, Taur-na-Chardhîn and Dor Dhínen would hardly be
explainable in that alternate scenario
Noldorin
dîn
noun. opening, gap, pass in mountains
dîn
noun. opening, gap, pass in mountains
A noun in The Etymologies of the 1930s glossed “opening, gap, pass in mountains”, the only derivative of ᴹ√DEN “hole, gap, passage” (Ety/DEN). It was an element in the names N. Din-Dûhir and N. Din-Caradras, but neither of these names appeared in the narratives.