A Doriathrin adjective meaning “secret, veiled” derived from root ᴹ√MUY (Ety/MUY), apparently a combination of muil “vagueness” and the adjective-suffix -in.
Doriathrin
muil
noun. twilight, shadow, vagueness
muilin
adjective. secret, veiled
dûm
noun. twilight
A noun for “twilight” developed from primitive ᴹ✶dōmi (Ety/DOMO), where the long [[ilk|[ō] became [ū] before [m]]].
A Doriathrin noun meaning “twilight, shadow, vagueness” derived from root ᴹ√MUY (Ety/MUY). Its Quenya cognate ᴹQ. muile suggests a primitive form of ✱✶muı̯lē [mujlē].
Conceptual Development: Tolkien seems to have coined this word to explain Umboth Muilin “Twilight Meres”, a remnant of earlier G. Umboth-muilin “Pools of Twilight”. The meaning of the Ilkorin elements is reversed from the earlier name: in the Gnomish name muilin “pools” was the plural form of the word G. muil, glossed “tarn” in the Gnomish Lexicon (GL/58) which is an older English word for “(mountain) lake”. In later writings, Tolkien replaced this name with S. Aelin-uial “Meres of Twilight”.