Where does “эфи́р” come from?

эфи́р (Russian) comes from Ancient Greek Αἰθήρ, from Ancient Greek αἴθω, from Proto-Hellenic *áitʰō.

Ancestry of “эфи́р”, step by step

Step	Language	Word	Meaning
1	Ancient Greek	Αἰθήρ	Upper Air
2	Ancient Greek	αἴθω	To ignite, kindle, light; To burn, blaze
3	Proto-Hellenic	*áitʰō	—