fíacail

See also: fiacail

Old Irish

Unknown. Michael Weiss has expressed a complete lack of faith that any satisfactory etymology can be found: "No etymology worth its salt."^[1]

Discredited etymologies

An ancient etymology derives it from Latin fīgō (“to fix”), the idea being that a tooth is a "mouth fixture";^[2] this explanation is impossible on phonological grounds and extremely unlikely on semantic ones.
MacBain suggests a connection with Middle Irish fec (“spade, tooth, tusk”),^[3] assuming that the latter is actually *féc; the two would then both be from Proto-Celtic *weikkā of unknown origin.^[4] This etymology is also phonetically impossible because Proto-Celtic *ei should have changed to ía before an unpalatalized consonant, so Middle Irish fec cannot have been **féc in the first place.

fíacail m or f (genitive fíacla or fíaclu, nominative plural fíaclai)

tooth
- c. 800–825, Diarmait, Milan Glosses on the Psalms, published in Thesaurus Palaeohibernicus (reprinted 1987, Dublin Institute for Advanced Studies), edited and with translations by Whitley Stokes and John Strachan, vol. I, pp. 7–483, Ml. 56^d4
  húa détnaig a fíaclae fri alailiu
  by the gnashing of their teeth against each other

Unknown gender i-stem
	Singular	Dual	Plural
Nominative	fíacail	fíacail^L	fíaclai^H
Vocative	fíacail	fíacail^L	fíaclai^H
Accusative	fíacail^N	fíacail^L	fíaclai^H
Genitive	fíaclo^H, fíacla^H	fíaclo^H, fíacla^H	fíaclae^N
Dative	fíacail^L	fíaclaib	fíaclaib
Initial mutations of a following adjective: ^H = triggers aspiration ^L = triggers lenition ^N = triggers nasalization

Mutation of *fíacail*
radical	lenition	nasalization
fíacail	ḟíacail	fíacail pronounced with /β(ʲ)-/

Note: Certain mutated forms of some words can never occur in Old Irish.
All possible mutated forms are displayed for convenience.

^ Weiss, Michael (2012) “Interesting i-stems in Irish”, in Adam I. Cooper, Jeremy Rau and Michael Weiss, editors, Multi Nominis Grammaticus: Studies in Classical and Indo-European linguistics in honor of Alan J. Nussbaum on the occasion of his sixty-fifth birthday, Ann Arbor, New York: Beech Stave Press, page 346
^ Gregory Toner, Sharon Arbuthnot, Máire Ní Mhaonaigh, Marie-Luise Theuerkauf, Dagmar Wodtko, editors (2019), “fíacail”, in eDIL: Electronic Dictionary of the Irish Language
^ Gregory Toner, Sharon Arbuthnot, Máire Ní Mhaonaigh, Marie-Luise Theuerkauf, Dagmar Wodtko, editors (2019), “fec”, in eDIL: Electronic Dictionary of the Irish Language
^ MacBain, Alexander, Mackay, Eneas (1911) “fíacail”, in An Etymological Dictionary of the Gaelic Language‎^[1], Stirling, →ISBN, page 172