Where does “certo” come from?

certo (Old Portuguese) comes from Latin certus, from Proto-Indo-European krey- — to sift, separate, divide.

Ancestry of “certo”, step by step

Step	Language	Word	Meaning
1	Latin	certus	certain; fixed, settled, firm; resolved,...
2	Proto-Indo-European	krey-	to sift, separate, divide

certo — Etymology, Origin & Word History | Etymology Explorer