One of the havens of Falas along with Brithombar (S/58). This name is apparently a combination of the prefixal form Egla- of Eglan “Forsaken (Elf)” and rest “a cut”, derived from ancient ✶(h)ekla-rista (WJ/365).
Conceptual Development: In Silmarillion drafts from the 1930s, this river was first named Eldorest after the river Eldor, revised to Ilk. Eglorest when the river name changed to Ilk. Eglor (SM/227, 310 note #8). In The Etymologies, this name was designated Ilkorin, related to Ilk. Egla “Elf” and rest “a cut” (Ety/ELED, RIS²). After Tolkien abandoned the Ilkorin language, the meaning of the initial element revised in his Quendi and Eldar essay from 1959-60, as given above (WJ/379). In this late essay, Tolkien did not explain the meaning of the second element, but it was clearly from the same primitive form as earlier Ilk. rest and may have retained the same meaning.
A name the Sindar used for themselves, mostly used in the plural forms Eglath, Eglain or Egladhrim (S/58, MR/170, WJ/365). It was often applied more specifically to the people of Círdan (WJ/380, PM/392 note #35). This name is derived from the adjective eglan “forsaken” < ✶heklanā, whereas the prefixal form Egla- (surviving only in names) was derived from the primitive noun form ✶hek(e)lā “a waif or outcast” (WJ/365). Tolkien experimented with a large variety of similar forms in Notes on Names from 1957, including eglon (PE17/140-142); see that entry for details.
Conceptual Development: In the Gnomish Lexicon from the 1910s, the word G. Egla was given as the Gnomish name for all Elves, meaning “a being from outside” and related to the word G. edh “outside” (which took the form eg before l) and ᴱQ. Elda (GL/32; LT1A/Eldar, Eglamar). Its class-plural Eglath also appeared in the Gnomish Lexicon (GL/50, entry idhrin). When Tolkien revised the phonology of the Noldorin language, he switched the language of this word to Ilk. Egla “Elf”, equivalent of ᴹQ. Elda and N. Eledh (Ety/ELED).
After Tolkien abandoned the Ilkorin language, he revised the meaning of this name to “The Forsaken”, a name the Sindar gave themselves after they were left in Beleriand (S/58). In this new sense, the class-plural S. Eglath “Forsaken” first appeared in a chart of the divisions of the Elves from the early 1950s, where it was equated to (and possibly replaced) Q. Ecelli of the same meaning (MR/170). The singular form Eglan appeared in his Quendi and Eldar essay from 1959-60, equated to Q. Hekel and with the etymology discussed above (WJ/365).