A lake in northwest Beleriand (S/106) named after the Elves who lived there (WJ/378). This name was the inspiration for Q. Sindar (PE17/140), and is a combination of mith “grey” and the class-plural suffix -rim (SA/mith, rim).
Conceptual Development: In the earliest Lost Tales, this lake was called G. Asgon (L1T1/238, GL/20), revised to ᴱN. Mithrim towards the end of the tales (LT2/202). The form N. Mithrim appeared in Silmarillion drafts from the 1930s (SM/4, LR/249), and also in The Etymologies, but with its first element being N. mith “white fog, wet mist” and its second element N. rhim “cold pool or lake (in mountains)”, hence “✱Mist Lake” (Ety/MITH, RINGI). The derivation from the name of the people came later (WJ/378), perhaps inspired in the real world by Q. Sindar, the reverse of the inspiration in the fictional world.
A Sindarin equivalent of Q. Sindar (VT41/9), a combination of thind “grey” and the class-plural suffix -rim.