Middle English word certein comes from Latin -anus, and later Latin *certanus (Certain.)

Detailed word origin of certein

Dictionary entry	Language	Definition
-anus	Latin (lat)	Of or pertaining to; -ian; usually indicates a relationship of position, possession, or origin.
*certanus	Latin (lat)	Certain.
certain	Old French (fro)	Certain; sure.
certein	Middle English (enm)	Certain.

Words with the same origin as certein