A prefix of Old English origin meaning "against", "back", "in return", "away", represented in Modern English by a-, an-, on-, and in altered form by the reverse-action prefix un- (e.g. unbuckle). Also as the initial letter d in dread (< Old English ondrǣdan).
Used in the sense of no or none, to show lack of or failure to perform; or in the sense of not, to negate the meaning of the word to which it is prefixed.
prevocalic form of nona-