"one not belonging to a regular body" of any sort, "one not subject to or not conforming with established regulations," 1610s, from irregular (adj.). Main modern sense of "a soldier not of the regular army" is from 1747.
Doubtless, the life of an Irregular is hard; but the interests of the Greater Number require that it shall be hard. If a man with a triangular front and a polygonal back were allowed to exist and to propagate a still more Irregular posterity, what would become of the arts of life? Are the houses and doors and churches in Flatland to be altered in order to accommodate such monsters? [Edwin Abbot, "Flatland: A Romance of Many Dimensions," 1885]
"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.
1570s, "unevenness;" 1660s, "deviation from the common rule," from Latin anomalia, from Greek anomalia "inequality," abstract noun from anomalos "uneven, irregular," from an- "not" (see an- (1)) + homalos "even," from homos "same" (from PIE root *sem- (1) "one; as one, together with"). From 1722 as "something abnormal or irregular."