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

Detailed word origin of certo

Dictionary entry	Language	Definition
cerno	Latin (lat)	I decide. I distinguish, discern, see. I perceive. I separate, sift.
*krinō	Proto-Italic (itc-pro)	Decide. Distinguish, discern. Sift, separate.
*krey-	Proto-Indo-European (ine-pro)
cretus	Latin (lat)
certo	Italian (ita)	(preposition) some, certain. Certain, sure. Reliable Certainly, of course, surely, definitely.