(n-)dôr(“land, dwelling-place”), dîn (“silence”) + en (adjective suffix) #The lack of lenition might be explained by Tolkien's dislike for “uncouth” digraph dh.
Sindarin
dor dínen
place name. Silent Land
Dor Dínen
noun. silent land
Dor Dínen
place name. Silent land
An uninhabited land in Beleriand, translated “Silent Land” (S/121, WJ/194), a combination of dôr “land” and dínen “silent” (SA/dîn).
Conceptual Development: In Silmarillion revisions from the 1950s-60s, Tolkien also used the form Dor Dhínen (WJ/333), reflecting uncertainty over the proper form of the adjective dínen.