fíadu

Old Irish

From Proto-Celtic *weidwūs, from *weyd- (“to know”) +‎ *-wōs (stative participle suffix). The n-stem inflection is secondary.^[1]

fíadu m

Masculine n-stem
	Singular	Dual	Plural
Nominative	fíadu	fíadain^L	fíadain
Vocative	fíadu	fíadain^L	fíadna^H
Accusative	fíadain^N	fíadain^L	fíadna^H
Genitive	fíadan	fíadan^L	fíadan^N
Dative	fíadain^L, fíadu^L	fíadnaib	fíadnaib
Initial mutations of a following adjective: ^H = triggers aspiration ^L = triggers lenition ^N = triggers nasalization

Old Irish mutation
Radical	Lenition	Nasalization
fíadu	ḟíadu	fíadu pronounced with /β(ʲ)-/
Note: Some of these forms may be hypothetical. Not every possible mutated form of every word actually occurs.

^ Stüber, Karin (1998) The Historical Morphology of N-stems in Celtic (Maynooth studies in Celtic linguistics), Department of Old Irish, National University of Ireland, Maynooth, →ISBN, page 115