1812, from French anormal, from Medieval Latin anormalus, a corruption of anomalus (from Greek anomalos "uneven, irregular;" see anomaly), as if from Latin ab "away" + norma "rule, pattern." In 20c., used as "an antonym of 'normal' when the association of 'abnormal', i.e. 'unhealthy', 'unnatural', would be inappropriate" [OED].
