Where does “Jyeshta” come from?

Jyeshta (English) comes from Sanskrit ज्येष्ठ, from Proto-Indo-Iranian *ȷ́yáHištʰas.

Ancestry of “Jyeshta”, step by step

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