A word for a “silver-birch”, an element in the names Nimbrethil and Fimbrethil (SA/brethil). According to Tolkien it was associated with bereth “queen”:
> ... since this tree was an emblem of Elbereth, was associated by Elves with the stars, and the word by them often interpreted as “daughter of the Queen, princess”. Fuller forms nimbrethil “white princess” and fimbrethil “slender princess” were also used (PE17/23).
Tolkien went on to add that “the ordinary non-mythological word for birch was ... S chwind, whinn”, so it seems brethil was only for the specific species of birch associated with Elbereth, and the ordinary word for “birch” was hwinn. In this note, Tolkien also derived brethil from √BARATH, but he may have changed his mind later; see the entry to bereth “queen” for discussion.
Conceptual Development: In The Etymologies of the 1930s, N. brethil was “beech (tree)” and it was derived from the root ᴹ√BERETH of the same meaning (Ety/BERETH, NEL). In later writings, “beech” was S. neldor; see that entry for discussion.
Forest where the Men of the House of Haleth dwelled in Beleriand, often called the Forest of Brethil (S/147). It is simply brethil “silver-birch” used as a name (Ety/NEL).
Conceptual Development: This name appeared in the Silmarillion drafts from the 1930s (SM/115 note #11, LR/261). N. Brethil appeared in The Etymologies, along with a variant N. Brethorn “Forest of Brethil” (Ety/NEL, EtyAC/NEL).