Where does “ith-” come from?

ith- (Old Irish) comes from Proto-Celtic ati-, from Proto-Indo-European éti, from Proto-Indo-European h₁é, from Proto-Indo-European ǵʰe — Postpositional intensifying particle: indeed, at any rate, in fact.

Ancestry of “ith-”, step by step

StepLanguageWordMeaning
1Proto-Celticati-re-, again, second, further
2Proto-Indo-Europeanétibeyond; over
3Proto-Indo-Europeanh₁éand, then
4Proto-Indo-EuropeanǵʰePostpositional intensifying particle: indeed, at any rate, in fact
Every word from Proto-Indo-European ǵʰe →Every word from Proto-Indo-European h₁é →Every word from Proto-Indo-European éti →