1. un- a prefix meaning “not,” freely used as an English formative, giving negative or opposite force in adjectives and their derivative adverbs and nouns (unfair; unfairly; unfairness; unfelt; unseen; unfitting; unformed; unheard-of; un-get-at-able), and less freely used in certain other nouns (unrest; unemployment).
2. un- a prefix freely used in English to form verbs expressing a reversal of some action or state, or removal, deprivation, release, etc. (unbend; uncork; unfasten; etc.), or to intensify the force of a verb already having such a meaning (unloose).
Etymology: (ME, OE un-, on-; c. D ont-=, G ent-; akin to L ante, Gk anti; cf. ante-, anti-)
1. un- used with adjectives, adverbs, and verbs to indicate the opposite meaning unhappy; unhook; unbelievably
Definition of 'un-'
Webster Dictionary
1. (adverb)un- an inseparable prefix, or particle, signifying not; in-; non-. In- is prefixed mostly to words of Latin origin, or else to words formed by Latin suffixes; un- is of much wider application, and is attached at will to almost any adjective, or participle used adjectively, or adverb, from which it may be desired to form a corresponding negative adjective or adverb, and is also, but less freely, prefixed to nouns. Un- sometimes has merely an intensive force; as in unmerciless, unremorseless
2. (adverb)un- un- is prefixed to adjectives, or to words used adjectively
3. (adverb)un- to adjectives, to denote the absence of the quality designated by the adjective
4. (adverb)un- to past particles, or to adjectives formed after the analogy of past particles, to indicate the absence of the condition or state expressed by them
5. (adverb)un- to present particles which come from intransitive verbs, or are themselves employed as adjectives, to mark the absence of the activity, disposition, or condition implied by the participle; as, - ---- and the like
6. (adverb)un- those which have acquired an opposed or contrary, instead of a merely negative, meaning; as, unfriendly, ungraceful, unpalatable, unquiet, and the like; or else an intensive sense more than a prefixed not would express; as, unending, unparalleled, undisciplined, undoubted, unsafe, and the like
7. (adverb)un- those which have the value of independent words, inasmuch as the simple words are either not used at all, or are rarely, or at least much less frequently, used; as, unavoidable, unconscionable, undeniable, unspeakable, unprecedented, unruly, and the like; or inasmuch as they are used in a different sense from the usual meaning of the primitive, or especially in one of the significations of the latter; as, unaccountable, unalloyed, unbelieving, unpretending, unreserved, and the like; or inasmuch as they are so frequently and familiarly used that they are hardly felt to be of negative origin; as, uncertain, uneven, and the like
8. (adverb)un- those which are anomalous, provincial, or, for some other reason, not desirable to be used, and are so indicated; as, unpure for impure, unsatisfaction for dissatisfaction, unexpressible for inexpressible, and the like
9. (adverb)un- un- is prefixed to nouns to express the absence of, or the contrary of, that which the noun signifies; as, unbelief, unfaith, unhealth, unrest, untruth, and the like
10. un- an inseparable verbal prefix or particle. It is prefixed: (a) To verbs to express the contrary, and not the simple negative, of the action of the verb to which it is prefixed; as in uncoil, undo, unfold. (b) To nouns to form verbs expressing privation of the thing, quality, or state expressed by the noun, or separation from it; as in unchild, unsex. Sometimes particles and participial adjectives formed with this prefix coincide in form with compounds of the negative prefix un- (see 2d Un-); as in undone (from undo), meaning unfastened, ruined; and undone (from 2d un- and done) meaning not done, not finished. Un- is sometimes used with an intensive force merely; as in unloose
Definitions of un- 
