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Published 01/10/24
Fèinn f. [fẽːɲ], with the article an Fhèinn [ə ˈɲẽːɲ], gen. na Fèinne [nə ˈfẽːɲ(ə)], a normalised dative form of an Fhiann [ə ˈɲĩə̃ᵰ̪] ‘the Fian, the Fingalians (see EG fían, below)’. Zimmer (1891, 15–16) takes Ir. fiann to be a grammatically singular feminine ā-stem formed via back-formation from a plural fianna, derived from ON fiandr [sic] (pl.). This is refuted by Marstrander (1915a, 123), who notes the word is really Irish. Wagner (1976, 264–66) is undecided: ‘Zimmer erklärte fian, pl. fianna aus anord. fiándi ‘Feind’, ob mit Recht, sei dahingestellt’; McDonald (2009, 353) considers the derivation unlikely.
ON fjándi m. nd-stem, (obl.) fjánda, pl. (nom./acc.) fjándr, (gen.) fjánda, (dat.) fjándum ‘enemy; fiend, devil’ (based on the verb fjá ‘to hate’) goes back to CSc. fíandi, before the shift of stress (Iversen 1973, 27). An oblique CSc. fíanda would formally be expected to yield an Early Gaelic form with an unlenited nasal, e.g. EG *fíänna (with hiatus) or *fínna (with a long monophthong 
Cf. SG Dìobadal NB549546 < CSc. *Diupadal acc. ‘(the) deep valley’, but SG (Loch) Diùbaig [(ˌɫ̪ɔx) ˈd̥ʲuːb̥ɛɡ̊ʲ] ‘(the loch of) Diùbaig’ NG323541 < ON *Djúp-vík ‘(the) deep-bay’ (Cox 2022, 677–80).
 ) or possibly *fíanna (with the diphthong /iə/), but not with a lenited nasal as in EG fían f. ā-stem ‘a band of roving men whose principal occupations were hunting and war; later esp. the warrior-bands under Finn mac Cuṁaill (SG Fionn mac Cumhaill)’; ON fjánda obl. might be expected to yield SG *feànn *[fjɑ̃:ᵰ̪] or *feann *[fjãũᵰ̪].
Stokes (1894, 265) forces a distinction between EG fían ‘hero’ and fíann ‘band of warriors’, so also MacBain (1896) and d’Arbois de Jubainville (1907, 249 fn 3), but this is artificial: the singulative sense is late and rare and not restricted to fían (eDIL˄). Wagner (ibid.) takes the view that EG fían contains unlenited nn, so that it should be seen as representing fíann, as it was later written. However, Meyer (≈1910, v–vi) explains that ‘[t]hroughout the centuries the form with one n continues to be the ordinary spelling of the best scribes, though nn begins to appear early in plural forms. Perhaps it was introduced here from the adjective fíanna (for older fíanḋae). But that for a long time, however it was written by individual scribes, it continued to be pronounced with a single n is proved by such rhymes as ... fíann : dian : riaṁ : fíaḋ : aníar [etc.].’ The nasal of the personal name Finn (Fionn) is perhaps also a conceivable influence on the development of (singular) fían to fíann.
Loth (1892, 506–08) sees a parallel between EG fíann and Bret. gouenn ‘breed, seed, extraction’ < *vein-nā or *vein-dā.
Hypothetical forms are given as cited.
Stokes (1894, 265) gives fían < *veino-s and fíann < *veinnā; so also MacBain (1896), while d’Arbois de Jubainville (1907, 249 fn 3) gives fían < *uēno-s. Meyer (1910, v–vi) agrees with Stokes’s equation of fían with Lat. *vēnā (hence Lat. vēnāri ‘to hunt’), both going back to Proto-Indo-European *u̯ei̯enā-; this is supported by Marstrander in a personal communication (ibid., vi fn 1). McCone (1987, 111) suggests a connection with Proto-Celtic *wēdu- ‘wild’, while Matasović (2009, 412–13) derives fían from Proto-Celtic *wēnā ‘troop, band of a warriors’.