A Doriathrin noun meaning “large pool”, apparently an elaboration of moth “pool” (Ety/MBOTH). Either the u- is some kind of prefix, perhaps an intensifier, or the initial [m] of the root became syllabic, both as suggested by Helge Fauskanger (AL-Ilkorin/umboth).

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: ᴱN./G. umboth meant “nightfall” or “twilight” (GL/75; PE13/155); see that entry for discussion. In later writings, Tolkien replaced this name with S. Aelin-uial “Meres of Twilight”.

A word appearing as G. umboth or umbath “nightfall” in the Gnomish Lexicon of the 1910s as an elaboration of the early root ᴱ√m(b)aþ- “dusk” (GL/75). It reappeared in ᴱN. umboth “twilight” in Early Noldorin word-lists (PE13/155). In the 1910s and 20s its most notable was in the name ᴱN./G. Umboth-muilin “Pools of Twilight” or “Twilight Meres” (LT2/225; LB/214), but in the 1930s Ilkorin form of this name Umboth Muilin the meaning of the first element umboth was “large pool” (Ety/MBOTH). In Tolkien’s later writings this name became Aelin-uial.