From Latin 'certus' (decided), from 'cernere' (to sift) — certainty is the result of sifting until truth remains.
Known for sure; established beyond doubt; having complete confidence or conviction about something.
From Old French 'certain,' from Vulgar Latin *certānus, from Latin 'certus' (determined, resolved, fixed, sure), the past participle of 'cernere' (to sift, to separate, to distinguish, to decide), from PIE *krey- (to sift, to separate). The deepest meaning is striking: to be certain is to have sifted the evidence and separated truth from falsehood. Certainty is not passive confidence but
The words 'certain,' 'crime,' 'crisis,' 'critic,' and 'secretary' all descend from the same PIE root *krey- (to sift/separate). A critic separates good from bad; a crisis is a moment that separates outcomes; a crime is a judgment that separates guilt from innocence; and a secretary was originally one who handled separated (secret) matters.