ᴹ√MUY root. “*secret”
An unglossed root in The Etymologies of the 1930s with derivatives like ᴹQ. muile “secrecy” and Ilk. muil “twilight, shadow, vagueness”, as well as Ilk. muilin “secret, veiled” in Ilk. Umboth Muilin “Veiled Pool” (Ety/MUY). Tolkien also gave N. muin “secret”, but he crossed that out and said the root was “not in N because it became identical with moina [MOY]” (Ety/MUY; EtyAC/MUY). Tolkien later abandoned the name Umboth Muilin, but S. muil reappeared in Emyn Muil “Drear Hills” (LotR/373; RC/334), hinting that this root may have remained valid.
References ✧ Ety/MBOTH, MUY