Where does “ζέστη” come from?

ζέστη (Greek) comes from Ancient Greek ζέω, from Proto-Indo-European yes- — to foam; to boil.

ζέστη (Greek): heat

Definitions

Step	Language	Word	Meaning
1	Ancient Greek	ζέω	to boil, seethe, bubble; to heat
2	Proto-Indo-European	yes-	to foam; to boil

ζέστη — Etymology, Origin & Word History | Etymology Explorer