Etymology
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cert 

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.

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short (adj.)

Old English sceort, scort "short, not long, not tall; brief," probably from Proto-Germanic *skurta- (source also of Old Norse skorta "to be short of," skort "shortness;" Old High German scurz "short"), from PIE root *sker- (1) "to cut," on the notion of "something cut off" (compare Sanskrit krdhuh "shortened, maimed, small;" Latin curtus "short," cordus "late-born," originally "stunted in growth;" Old Church Slavonic kratuku, Russian korotkij "short;" Lithuanian skursti "to be stunted," skardus "steep;" Old Irish cert "small," Middle Irish corr "stunted, dwarfish," all from the same root).

Meaning "having an insufficient quantity" is from 1690s. Meaning "rude" is attested from late 14c. Meaning "easily provoked" is from 1590s; perhaps the notion is of being "not long in tolerating."

Short fuse in figurative sense of "quick temper" first attested 1968. To fall short is from archery. Short run "relatively brief period of time" is from 1879. Short story first recorded 1877. Short cut is from 1580s, from cut (n.) in the sense "passage, course, or way straight across" (1570s). To make short work of "dispose of quickly" is first attested 1570s. Phrase short and sweet is from 1530s. To be short by the knees (1733) was to be kneeling; to be short by the head (1540s) was to be beheaded.

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certain (adj.)

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.

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