irregular (adj.) Look up irregular at Dictionary.com
late 14c., "not in conformity with Church rules," from Old French irreguler (13c., Modern French irrégulier), from Medieval Latin irregularis, from assimilated form of in- "not, opposite of" (see in- (1)) + Latin regularis (see regular (adj.)). General sense is from late 15c.
irregular (n.) Look up irregular at Dictionary.com
"a soldier not of the regular army," 1747, from irregular (adj.).