Where does “जेठउं” come from?

जेठउं (Old Gujarati) comes from Sanskrit ज्येष्ठ, from Proto-Indo-Iranian *ȷ́yáHištʰas.

जेठउं (Old Gujarati): eldest

Definitions

Step	Language	Word	Meaning
1	Sanskrit	ज्येष्ठ	most excellent, pre-eminent, first, chief, best,...
2	Proto-Indo-Iranian	*ȷ́yáHištʰas	—

जेठउं — Etymology, Origin & Word History | Etymology Explorer