c. 1300, "determined, fixed," from Old French certain "reliable, sure, assured" (12c.), from Vulgar Latin *certanus, extended form of Latin certus "determined, resolved, fixed, settled," of things whose qualities are invariable, "established," also "placed beyond doubt, sure, true, proved; unerring, to be depended upon" (also source of Old French cert, Italian certo, Spanish cierto), originally a variant past participle of cernere "to distinguish, decide," literally "to sift, separate." This Latin verb comes from the PIE root *krei- "to sieve," thus "discriminate, distinguish," which is also the source of Greek krisis "turning point, judgment, result of a trial" (compare crisis).
Transferred sense, in reference to persons, "full of confidence in one's knowledge or judgment, made certain in reference to a matter or thing," from mid-14c. (also a sense in Latin). Meaning "established as true beyond doubt" in English is from c. 1400. Meaning "indefinite, not specifically named, known but not described" is from late 14c.
Different as this seems to be from sense I, it is hardly separable from it in a large number of examples: thus, in [a certain hour], the hour was quite 'certain' or 'fixed', but it is not communicated to the reader; to him it remains, so far as his knowledge is concerned, quite indefinite; it may have been, as far as he knows, at any hour; though, as a fact, it was at a particular hour. [OED]
Lewis & Short write that Latin certus also was sometimes indefinite, "of things, the certainty of whose existence is given, but whose nature is not more definitely designated, or comes not into consideration ...."
Hence the euphemistic use, attested from mid-18c., as in woman of a certain age "an old maid;" woman of a certain description "disreputable woman;" in a certain condition "pregnant;" a certain disease "venereal disease;" of a certain weight "obese." Used with proper names from 1785, "often conveying a slight shade of disdain" [OED]. 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.
"certainly, verily, in truth," mid-13c., from Old French certes, from Vulgar Latin *certas, from Latin certe, adverb from certus "fixed, certain" (see certain).
"certainty, complete assurance," early 15c., from Old French certitude "certainty" and directly from Late Latin certitudinem (nominative certitudo) "that which is certain," from Latin certus "sure, certain" (see certain).
c. 1300, certeynte, "surety, pledge," from Anglo-French certeinté (late 13c.), Old French certainete "certainty," from Latin or Vulgar Latin *certanitatem (source of Old Spanish certanedad), from Vulgar Latin *certanus (see certain). Meaning "that which is certain, a clear fact or truth" is attested from early 14c.; meaning "quality or fact of being certain; full assurance of mind, exemption from doubt" is from early 15c.
"writ from superior to inferior courts seeking the records of a case," legal Latin, "to be certified, to be informed or shown," a word figuring in the opening phrase of such writs; passive present infinitive of certorare "to certify, inform," from certior, comparative of certus "sure" (see certain).
mid-14c., "to declare the truth of," also "to vouch for or confirm" (an official record, etc.), from Old French certefiier "make certain, witness the truth of" (12c.), from Late Latin certificare "to certify, to make certain," from Latin certus "fixed, sure" (see certain) + root of facere "to make, do" (from PIE root *dhe- "to set, put"). Also used in Middle English in broader senses of "inform, give notice to; instruct, to direct; to designate." Related: Certified; certifying.
mid-15c., "variability," from Old French incertitude (14c.), from Late Latin incertitudinem (nominative incertitudo) "uncertainty," from in- "not, opposite of" (see in- (1)) + certitudo "that which is certain," from Latin certus "sure, certain" (see certain). From c. 1600 as "doubt, hesitation." Middle English also had incertain "uncertain" and incertainty "uncertainty," both from Old French, but both have been displaced by forms in un-.