Latin word certus comes from Proto-Italic *krinō (Decide. Distinguish, discern. Sift, separate.)

Detailed word origin of certus

Dictionary entry	Language	Definition
*krinō	Proto-Italic (itc-pro)	Decide. Distinguish, discern. Sift, separate.
cernere	Latin (lat)
*krey-	Proto-Indo-European (ine-pro)
certus	Latin (lat)	Certain. Fixed, settled, firm. Resolved, determined. Sure.