"not conformed or conforming to rule, deviating from a type or standard, contrary to system or law, irregular, unnatural," 1835, a refashioning of anormal (q.v.) 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- "not" (see an- (1)) + homalos "even," from homos "same" (from PIE root *sem- (1) "one; as one, together with"). Compare anomaly. "Few words show such a series of pseudo-etymological perversions." Another adjective was abnormous (1742) "irregular, misshapen," from Latin abnormis. Related: Abnormally.