mid-14c., "evil, an evil act," also " a trifle," c. 1400, "nothingness;" early 15c., in arithmetic, "the number zero;" from noht, naht (pron.) "nothing" (late 12c.), from Old English nawiht "nothing," literally "no whit," from na "no" (from PIE root *ne- "not") + wiht "thing, creature, being" (see wight). Also see nought.

Cognate with Old Saxon neowiht "nothing," Old High German niwiht, Gothic ni waihts, Dutch niet, German nicht. It also developed an adjectival sense in Old English, "good for nothing," which by mid-16c. had focused to "morally bad, wicked," though the modern adjective is naughty.

a way of writing can not, c. 1400, from can (v.1) + not. Old English expressed the notion by ne cunnan. The typical representation of the Scottish pronunciation is canna.

by 1670s, contraction of could + not.

"poor person," 1742, from have + not. Have in the sense of "one who 'has,' one of the wealthier class of persons" is from the same source. Earliest in translation of "Don Quixote:

'There are but two families in the world, as my grandmother used to say; "the Have's and the Have-not's," and she stuck to the former; and now-a-days, master Don Quixote, people are more inclined to feel the pulse of Have than of Know.' ["Don Quixote de la Mancha," transl. Charles Jarvis, London, 1742]

a negative present participle used as a quasi-preposition, originally and properly two words, late 14c., notwiþstondynge "in spite of, despite," from not + present participle of the verb withstand. It has the old "against" sense of with. A loan-translation of Medieval Latin non obstante "being no hindrance," literally "not standing in the way," from ablative of obstans, present participle of obstare "stand opposite to" (see obstacle). As an adverb, "nevertheless, however," and as a conjunction, "in spite of the fact that," from early 15c.

Notwithstanding ... calls attention with some emphasis to an obstacle: as, notwithstanding his youth, he made great progress. In spite of and despite, by the strength of the word spite, point primarily to active opposition: as, in spite of his utmost efforts, he was defeated; and, figuratively, to great obstacles of any kind: as, despite all hindrances, he arrived at the time appointed. [Century Dictionary]

also what-not, 1530s, "anything," from what + not. Elliptical for "what may I not say," implying "everything else." As the name of a furniture item, first attested 1808, so named for the objects it is meant to hold.

Proto-Indo-European root meaning "not."

It forms all or part of: a- (3) "not, without;" abnegate; ahimsa; an- (1) privative prefix; annihilate; annul; aught (n.2) "zero, nothing;" deny; hobnob; in- (1) "not, opposite of, without;" ixnay; naught; naughty; nay; nefarious; negate; neglect; negligee; negotiate; neither; nepenthe; nescience; nescient; neuter; never; nice; nihilism; nihility; nil; nill; nimiety; nix; no; non-; none; nonplus; nor; not; nothing; null; nullify; nulliparous; renegade; renege; un- (1) prefix of negation; willy-nilly.

It is the hypothetical source of/evidence for its existence is provided by: Sanskrit a-, an- "not;" Avestan na "not;" Greek a-, an-, ne- "not;" Latin in- "not," ne "that not;" Old Church Slavonic and Lithuanian ne "not;" Old Irish an-, ni, Cornish ny "not;" Gothic and Old English un- "not."

a prefix used freely in English and meaning "not, lack of," or "sham," giving a negative sense to any word, 14c., from Anglo-French noun-, from Old French non-, from Latin non "not, by no means, not at all, not a," from Old Latin noenum "not one" (*ne oinom, from PIE root *ne- "not" + PIE root *oi-no- "one, unique"). In some cases perhaps from Middle English non "not" (adj.), from Old English nan (see not). "It differs from un- in that it denotes mere negation or absence of the thing or quality, while un- often denotes the opposite of the thing or quality" [Century Dictionary].

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."

The most prolific of English prefixes, freely and widely used in Old English, where it forms more than 1,000 compounds. 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.

Often euphemistic (untruth for "a lie") or emphatic, if there is a sense already of divestment or releasing: unpeel " to peel;" unpick "pick (a lock) with burglars' tools;" unloose for "to loosen."

It also makes words from phrases, such as uncalled-for, c. 1600; undreamed-of, 1630s. Fuller (1661) has unbooklearned. A mid-15c. description of a legal will has unawaydoable; Ben Jonson has un-in-one-breath-utterable. The word uncome-at-able is attested by 1690s in Congreve, frowned at by Samuel Johnson in the 18th century and by Fowler in the 20th ("The word had doubtless, two or three centuries ago, a jolly daredevil hang-the-grammarians air about it ; that has long evaporated ; it serves no purpose that inaccessible does not ....").

But the practice continued; unlawlearned (Bentham, 1810), unlayholdable (1860); unputdownable, of a book, is by 1947; unpindownable, by 1966. Also compare put-up-able-with (1812). As a prefix in telegraphese, to replace not and save the cost of a word, it is attested by 1936.

With the variety of its possible use, and the need for negatives, the number of un- words that might be made in English is almost endless, and that some are used and some never is owing to the caprice of authors.

Dictionary editors noted this since 18c. but also padded the list. John Ash's "New and Complete Dictionary of the English Language" (1775) has many pages of one-line un- entries; among a dozen consecutive entries are unhaggled, unhaired, unhalooed, unhaltering (adj.), unhaltering (n.), which sorts of words OED (1989) notes were "obviously manufactured for the purpose" and some turn up in other texts only decades later, if at all.