A forest in Doriath (S/55) with the many beech trees (LotR/469). Its initial element is neldor “beech” (SA/neldor), and its final element appears to be the abstract noun ending -eth.
Conceptual Development: The name Ilk. Neldoreth appeared in Silmarillion drafts from the 1930s (LR/126), and was designated Doriathrin [Ilkorin] in The Etymologies, with the same derivation as given above (Ety/NEL).
A Sindarin word for “beech” appearing in the names Taur-na-Neldor “Beech-forest” (LotR/469; RC/384) and Neldoreth, the name of a forest with beeches (S/55; PE17/81).
Conceptual Development: In The Etymologies of the 1930s, Ilk. neldor was an Ilkorin word based on ᴹ√NÉL-ED “three”, which Tolkien said was “properly name of Hirilorn the great beech of Thingol with three trunks = neld-orn ? [question mark from Tolkien]” (Ety/NEL). In the 1910s and 20s, ᴱQ. neldor “beech” was an Early Qenya word (PE16/139; QL/65), and its cognates in this period were G. deldron “beech” in the Gnomish Lexicon of the 1910s (GL/30), G. deil(i)an or delwen “beech” in Gnomish Lexicon Slips (PE13/112), and ᴱN. {de(i)lian >>} deilian “beech-tree” in Early Noldorin Word-lists of the 1920s (PE13/142).
Possible Etymology: The Ilkorin derivation from √NELED is no longer suitable in Sindarin, since we would expect [[s|[d] > [ð]]] as in S. neledh “three”. It is possible Tolkien simply never reexamined the etymology of this word after it became Sindarin. Alternately, it could be nel- “tri-” + taur “forest” or -dor “-lord” or something similar.