prefix.
1 added to adjectives and participles and their derivative nouns and adverbs, meaning: a not: denoting the absence of a quality or state (unusable; uncalled-for; uneducated; unfailing; unofficially; unhappiness). b the reverse of, usu. with an implication of approval or disapproval, or with some other special connotation (unselfish; unsociable; unscientific).
Usage:
Words formed in this way often have neutral counterparts in non- (see NON- 6) and counterparts in -in (see IN-(1)), e.g. unadvisable.
2 (less often) added to nouns, meaning 'a lack of' (unrest; untruth).
Usage:
The number of words that can be formed with this prefix (and similarly with un-(2)) is potentially as large as the number of adjectives in use; consequently only a selection, being considered the most current or semantically noteworthy, can be given here.
Etymology: OE f. Gmc, rel. to L in-
2.
prefix added to verbs and (less often) nouns, forming verbs denoting:
1 the reversal or cancellation of an action or state (undress; unlock; unsettle).
2 deprivation or separation (unmask).
3 release from (unburden; uncage).
4 causing to be no longer (unman).
Usage:
See the note at un-(1). Both un-(1) and un-(2) can be understood in some forms in -able, -ed (especially), and -ing: for example, undressed can mean either 'not dressed' or 'no longer dressed'.
Etymology: OE un-, on- f. Gmc
Useful english dictionary. 2012.