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 with certain words to reverse their meaning
in, into
in, into, towards, within.
Added to adjectives to mean not
Added to nouns to mean lacking or without