colloquial abbreviation of certainty, attested by 1889 (in dead cert). Cert (adv.) "forsooth, indeed," was in Middle English, from Old French, from Latin certo, certe, but it became obsolete or dialectal.
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.
1846, "capable of being declared as true," from certify + -able. Meaning "so deranged as to be certifiably insane" is recorded from 1870, from certify in the specific sense "officially declare (someone) to be insane" (1822). The certification was done by local officials, later medical officers, and often included a statement as to whether the person was harmless or dangerous, curable or incurable.
"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).
"attested by certificate," 1610s, past-participle adjective from certify. Certified public accountant attested from 1896; certified mail from 1955.
"licensed or authorized by certificate," 1610s, past-participle adjective from obsolete certificate (v.) "furnish (someone) with a certificate," from Medieval Latin certificatus, past participle of certificare "to make certain" (see certify).
"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).
"certainly, verily, in truth," mid-13c., from Old French certes, from Vulgar Latin *certas, from Latin certe, adverb from certus "fixed, certain" (see certain).
early 15c., "notification;" mid-15c., "demonstration, proof," from Medieval Latin certificationem (nominative certificatio), noun of action from past-participle stem of Late Latin certificare "to make certain" (see certify). Meaning "act of providing with a legal certificate" is from 1881.