irregular (adj.)youdaoicibaDictYouDict[irregular 词源字典]
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 etymology, irregular origin, 英语词源]
irregular (n.)youdaoicibaDictYouDict
"a soldier not of the regular army," 1747, from irregular (adj.).