abnormal (adj.) Look up abnormal at Dictionary.com
1835, a refashioning of anormal (1835) under influence of Latin abnormalis "deviating from a fixed rule, irregular," from ab "off, away from" (see ab-) + norma "rule" (see norm).

The older form was from French anormal (13c.), from Medieval Latin anormalus, an altered (by association with norma) borrowing of Greek anomalos "uneven, irregular," from an-, privative prefix, "not" (see an- (1)) + homalos "even," from homos "same" (see same). Compare anomaly. "Few words show such a series of pseudo-etymological perversions." Another adjective was abnormous (1742) "irregular, misshapen," from Latin abnormis. Related: Abnormally.