A word for “mine, boring, tunnel, underground dwelling” in Notes on Names (NN) from 1957, but also explained in notes from 1969 as “minor excavations made by wild animals as dens or lairs” (NM/304). It was derived from primitive ✶phelgā (NM/304; PE17/118; Ety/PHÉLEG), and the final a in this word is the result of ancient ʒ (from g) become a when word-final after another consonant.
Abnormal Plural: This word has an abnormal plural form fili (NM/304; Ety/PHÉLEG): see the section on “Final a from ancient g” in the discussion of unusual plurals for more details.
Conceptual Development: This word was tied to the name of Felagund since its introduction in The Etymologies of the 1930s, where N. fela “cave” was derived from ON. phelga under the root ᴹ√PHELEG of the same meaning, already with the abnormal plural fili noted above (Ety/PHÉLEG). In Notes on Names (NN) from 1957, Tolkien again had S. fela from ✶phelgā, but there the gloss was “mine, boring, tunnel, underground dwel[ling]” (PE17/118). In this note Tolkien considered instead S. feleg “cave, mine, underground dwelling” as the basis for the name Felagund, derived instead from √PHELEK (PE17/118).
In a note from 1959, Tolkien gave a completely different etymology of Felagund as a loan word from Khuzdul Felakgundu “Cave Hewer” (PM/352), and this was the etymology Christopher Tolkien gave in The Silmarillion index (SI/Felagund). In a note from 1969, however, Tolkien said instead that Felagund was a nickname meaning “den-dweller” (also used for badgers), and its initial element fela was again derived from ✶phelga or philga (NM/304), with a meaning as follows:
> It was used of minor excavations made by wild animals as dens or lairs, and also as temporary dwellings by wandering folk, Dwarvish or Elvish; it was usually distinguished from the larger caves of geological formation used and extended by stone-workers. It was thus naturally used of the “setts” of badgers (which seem to have existed in great numbers in parts of Beleriand).
In this 1969 note Tolkien again mentioned its abnormal plural fili < ✶phelgai.
Neo-Sindarin: Of the various meanings for this name, I prefer its 1957 sense “mine, boring, tunnel, underground dwelling”; its 1969 use for “den” (and felagund = “badger”) conflicts with the etymology of Felagund’s published in The Silmarillion. For “cave” I would use groth as in Menegroth “Thousand Caves” or feleg as a loan word from Khuzdul, and for “den” I would use torech as in Torech Ungol “Shelob’s Lair”.
n. Q. felya. phelgā (the added vowel might be ō) << phelg-.