Where does “certán” come from?

certán (Old Irish) comes from Old Irish cert, from Latin certus, from Proto-Indo-European krey- — to sift, separate, divide.

Ancestry of “certán”, step by step

Step	Language	Word	Meaning
1	Old Irish	cert	correct, right, proper, fitting; fair, just;...
2	Latin	certus	certain; fixed, settled, firm; resolved,...
3	Proto-Indo-European	krey-	to sift, separate, divide

certán — Etymology, Origin & Word History | Etymology Explorer