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). °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'.
[ OE un-, on- f. Gmc ]