From: Old-Irish-L <[log in to unmask]> On Behalf Of Dennis King
> nathir (Sean-Ghaeilge) < *natrixs (Ceiltis) beithir (Sean-Ghaeilge) < *betrixs (Ceiltis) ??
The examples of beithir that show k-inflection don't have a lot of evidential weight. There is a tendency throughout the history of Irish to inflect as k-stems all words ending in -thir, this is spreading before our eyes. Forms like beithri without -ch, esp. in early texts, are more crucial. We can mechanically set up a Celtic preform *betrī or *bete/irī, or even *beturī, but that doesn't reveal anything insightful. The word is formally reminiscent of beithe 'birch' < *betu̯ii̯o-, but maybe that's just coincidence.