Where does “un deg tair” come from?

un deg tair (Welsh) comes from Welsh un, from Middle Welsh un, from Proto-Brythonic ʉn, from Proto-Celtic oinos, from Proto-Indo-European óynos — one; single.

Ancestry of “un deg tair”, step by step

un deg tair traces back through 3 separate lines of ancestry.

via Welsh un

StepLanguageWordMeaning
1Welshunonly; one; one, individual
2Middle Welshunone
3Proto-Brythonicʉnone
4Proto-Celticoinosone
5Proto-Indo-Europeanóynosone; single

via Welsh tair

StepLanguageWordMeaning
1Welshtairfeminine of tri; three
2Proto-Celtictisres—
3Proto-Indo-Europeantisresfeminine nominative/vocative of *tréyes
4Proto-Indo-Europeantrisres—
5Proto-Indo-Europeantréyesthree

via Welsh deg

StepLanguageWordMeaning
1Welshdegten; Soft mutation of teg
2Proto-Brythonicdegten
3Proto-Celticdekamten
4Proto-Indo-Europeandéḱm̥ten
Every word from Proto-Indo-European óynos →Every word from Proto-Celtic oinos →Every word from Proto-Brythonic ʉn →