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: in Gnomish the word G. umboth meant “twilight” (GL/75). In later writings, Tolkien replaced this name with S. Aelin-uial “Meres of Twilight”.