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”.
muilnoun. twilight, shadow, vagueness
A noun for “twilight” developed from primitive ᴹ✶dōmi (Ety/DOMO), where the long [[ilk|[ō] became [ū] before [m]]].