uncanny (adj.)

prefix of negation, Old English un-, from Proto-Germanic *un- (source also of Old Saxon, Old Frisian, Old High German, German un-, Gothic un-, Dutch on-), from PIE *n- (source of Sanskrit a-, an- "not," Greek a-, an-, Old Irish an-, Latin in-), combining form of PIE root *ne- "not." Often euphemistic (such as untruth for "lie").

The most prolific of English prefixes, freely and widely used in Old English, where it forms more than 1,000 compounds. It underwent a mass extinction in early Middle English, but emerged with renewed vigor 16c. to form compounds with native and imported words. It disputes with Latin-derived cognate in- (1) the right to form the negation of certain words (indigestable/undigestable, etc.), and though both might be deployed in cooperation to indicate shades of meaning (unfamous/infamous), typically they are not.

It also makes words from phrases (such as uncalled-for, c. 1600; undreamed-of, 1630s; uncome-at-able, 1690s; unputdownable, 1947, of a book; un-in-one-breath-utterable, Ben Jonson; etc., but the habit is not restricted to un-; such as put-up-able-with, 1812). As a prefix in telegramese to replace not and save the cost of a word, it is attested by 1936.

"knowing, wise," 1630s, from a Scottish and northern English formation from can (v.1) in its sense of "know how to," + -y (2). A doublet of cunning that flowered into distinct senses in Scottish English. In the glossary to Scott's "Heart of Mid-Lothian" (1818) uncanny is defined as "dangerous," while canny, as used in the tale, is defined as "skilful, prudent, lucky ; in a superstitious sense, good-conditioned, and safe to deal with ; trustworthy ; quiet." Cannily is "gently" and canny moment is "an opportune or happy time."

"Knowing," hence, from 18c., "careful, skillful, clever," also "frugal, thrifty," and, from early 19c. (perhaps via Scott's novels) "cautious, wary, shrewd." Often used superciliously of Scots by their southern neighbors (and their American cousins).

The Canny Scot is so well known as scarcely to require description. He carries caution, cunning, and selfishness to excess. Deceitful when a purpose is to be accomplished, he is not habitually deceitful. One thing he never loses sight of—his own interest. But of his own interest he is not the most enlightened judge. ["The Natural History of Scotsmen," in The Argosy, December 1865]

Related: Cannily; canniness.

early 14c., conning, "learned, skillful, possessing knowledge," present participle of connen, cunnen "to know," from Old English cunnan (see can (v.1)), from PIE root *gno- "to know." Also compare cun (v.). Sense of "skillfully deceitful, characterized by crafty ingenuity" is probably by late 14c. Related: Cunningly.