certainyoudaoicibaDictYouDict[certain 词源字典]
certain: [13] Certain comes ultimately from Latin certus ‘sure, fixed’, which derived from the past participle of the verb cernere ‘decide’. The Latin adjective was extended in Vulgar Latin to *certānus, which passed into English via Old French certain. Other English words based on certus include certify [14] (from late Latin certificāre) and its derivative certificate, and certitude [15] (from late Latin certitūdō).
=> crime, crisis, decree, discern, discrete, discriminate, excrement, riddle, secret[certain etymology, certain origin, 英语词源]
certain (adj.)youdaoicibaDictYouDict
c. 1300, "determined, fixed," from Old French certain "reliable, sure, assured" (12c.), from Vulgar Latin *certanus, from Latin certus "sure, fixed, settled, determined" (also source of Italian certo, Spanish cierto), originally a variant past participle of cernere "to distinguish, decide," literally "to sift, separate" (see crisis).

Of persons, "full of confidence in one's knowledge or judgment," from mid-14c. Euphemistic use (of a certain age, etc.) attested from mid-18c. Certainer, certainest were common to c. 1750, but have fallen from proper use for some reason. Expression for certain "assuredly" is attested by early 14c.