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.
_n. Bot. _silver birch, an emblem of Elbereth. Because of its association with Elbereth, Elves associated this tree with stars, and the word was often interpreted as 'daughter of the Queen', 'princess'. >> fimbrethil, nimbrethil