sanc·ti·mo·ni·ous (sāngk'tə-mō'nē-əs) adj. Feigning piety or righteousness: "a solemn, unsmiling, sanctimonious old iceberg that looked like he was waiting for a vacancy in the Trinity"(Mark Twain). sanc'ti·mo'ni·ous·ly adv., sanc'ti·mo'ni·ous·ness n.