Source of the river Narog (S/209) translated “Ivrin’s Well” (WJ/139), a combination of eithel “spring, well” and the name Ivrin (SA/eithel).
Conceptual Development: In Silmarillion drafts from the 1930s, this name first appeared as N. Ivrineithil with the same elements and meaning (SM/313, LR/139). However, in this earlier form, the second element eithil seems to be plural. Silmarillion drafts of from the 1950s likewise had plural eithil: Eithil Ivrin (WJ/85, 139). Christopher Tolkien chose to use singular Eithel in The Silmarillion as published (S/209).
A Sindarin noun for a source of water (RC/772). It appeared as an element in several names, such as Mitheithel, the Sindarin name for the river Hoarwell (LotR/200). From its glosses, it seems it could apply to a variety of water sources, including springs, wells and fountains (RC/772, SA/eithel, LR/301). It was derived from the primitive form ✶et-kelē, literally “✱out-flow”, but in ancient times the [tk] was transposed to [kt] giving ektelē (SA/kel; Ety/KEL). In Sindarin, this pair of voiceless stops became spirants [xθ], and then the [[s|[x] vocalized to [i]]] to form the diphthong [ei].
Conceptual Development: In the Gnomish Lexicon from the 1910s, there were two distinct precursors to this word, G. aithl “spring” (GL/18) and G. {ecthel >>} ectheluin “fountain, fount” (GL/31) or ecthelin (GL/25); the latter’s form was ecthel in various name lists form this period (PE13/104; PE15/23). In Early Noldorin Word-lists from the 1920s, there was an additional form ᴱN. eithlos “fountain” with several variants (PE13/142, 158); ᴱN. aithl “spring, fount, source” also reappeared (PE13/136, 158). In The Etymologies from the 1930s, these two forms seem to have merged into N. eithel “spring, issue of water” (Ety/KEL), and this is the source of derivation given above. All of the earlier forms seem to broadly have the same etymology, and so the changes represent the evolving phonetic development of the languages.