Where does “греть” come from?

греть (Russian) comes from Proto-Slavic grěti, from Proto-Indo-European gʷʰer-, from Proto-Indo-European gʷʰe-gʷʰór-e ~ gʷʰe-gʷʰr-ḗr.

Ancestry of “греть”, step by step

Step	Language	Word	Meaning
1	Proto-Slavic	grěti	to warm, to heat; to shine
2	Proto-Indo-European	gʷʰer-	warm, hot
3	Proto-Indo-European	gʷʰe-gʷʰór-e ~ gʷʰe-gʷʰr-ḗr	—

греть — Etymology, Origin & Word History | Etymology Explorer