Where does “calo” come from?

calo (Latin) comes from Proto-Indo-European ḱelh₁- — to warm up, to be hot; to warm up, be hot.

Ancestry of “calo”, step by step

Step	Language	Word	Meaning
1	Proto-Indo-European	ḱelh₁-	to warm up, to be hot; to warm up, be hot

calo — Etymology, Origin & Word History | Etymology Explorer