Doriathrin precursor to S. Aelin-uial translated “Twilight Meres” (LR/262). In The Etymologies, it was translated “Veiled Pool” and given as a combination of umboth “large pool” and muilin “veiled” (Ety/MBOTH, MUY).
Sindarin
Umboth Muilin
noun. veiled pool
umboth (Dor. “large pool”) + muilin (Dor. “secret, veiled”)