"not religious, without religious principles; condemning religion, impious, ungodly," c. 1400, from Late Latin irreligiosus "irreligious, impious," from assimilated form of in- "not, opposite of" (see in- (1)) + religiosus (see religious). Related: Irreligiously.
1590s, "irreligious, lacking reverence for God," from Latin impius "without reverence, irreverent, wicked; undutiful, unpatriotic," from assimilated form of in- "not, opposite of" (see in- (1)) + pius (see pious). Related: Impiously; impiousness.
late 14c., "irreligious, not god-fearing, not in accordance with the laws of God," from un- (1) "not" + godly (adj.). Similar formation in Middle Dutch ongodelijc, German ungöttlich, Middle Swedish ogudhlik. Colloquial sense of "extremely annoying" is recorded from 1887.
mid-14c., "acting falsely," from un- (1) "not" + faithful. In Middle English it also had a sense of "infidel, unbelieving, irreligious" (late 14c.). Sense of "not faithful in marriage" is attested from 1828. Related: Unfaithfully; unfaithfulness.