Where does “nírubtar” come from?

nírubtar (Old Irish) comes from Old Irish ní, from Proto-Celtic nīs, from Proto-Indo-European ne h₁ésti — is not.

Ancestry of “nírubtar”, step by step

nírubtar traces back through two separate lines of ancestry.

via Old Irish ní

StepLanguageWordMeaning
1Old Irishnínot; is not, isn’t; something, anything
2Proto-Celticnīs—
3Proto-Indo-Europeanne h₁éstiis not

via Old Irish batar

StepLanguageWordMeaning
1Old Irishbatarthird-person plural preterite/imperfect...
Every word from Proto-Indo-European ne h₁ésti →Every word from Proto-Celtic nīs →Every word from Old Irish ní →