The lake above the falls of Rauros, translated “Mist-cool Water” in Tolkien’s “Unfinished Index” of The Lord of the Rings (RC/327-8). This name is a combination of nen “water”, hîth “mist” and oel “cool” (SA/nen, hîth).
Conceptual Development: In Lord of the Rings drafts from the 1940s, Tolkien first considered the names N. Kerin-muil and Nen-uinel (TI/364), only later settling on Nen Hithoel.
It may be that when Tolkien first conceived of this name, the final element was N. oel “lake”. Prior to the publication of The Lord of the Rings: a Reader’s Companion, this was a common theory for the meaning of this name. When Noldorin became Sindarin, the word N. oel became S. ael, as in S. Aelin-uial “Meres of Twilight”, at which point Tolkien may have then revised the etymology for Nen Hithoel as given above. Since Nen Hithoel was a comparatively late composition, however, it may be that meaning “Mist-cool Water” was always Tolkien’s intent.
nen (“water”), hîth (“grey”) + oel (“cool”) [HKF] oel was reinterpreted by Tolkien from earlier “lake” when he decided that primitive ai yielded ae, not oe; #[His.] - “[hithoel] seems to show that words beginning with an h may resist the lenition”.