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).
incorrect
painful
unhealthy, harmful
abnormal
difficult
poor, deficient
to fail
inability, unable
malady, disease
not
bad