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.
exceptional adj.
1828, "out of the ordinary course, forming an exception, unusual," from exception + -al (1). Related: Exceptionally. Exceptionalism "fact or quality of being exceptional" in some way, usually implying superiority to the unexceptional, is attested from 1864; the phrase American exceptionalism is attested by 1929, originally among communists, in reference to the argument about whether the United States is in some sense not subject to the historical rules of Marxism; it has been used in other ways since, often implying (and implicitly criticizing) a belief that the U.S. is somehow uniquely virtuous. Other noun forms include exceptionalness (1868), exceptionality (1851).