c.1500, "resembling sauce," later "impertinent, flippantly bold, cheeky" (1520s), from sauce, n. + -y(2). The connecting notion is the figurative sense of "piquancy in words or actions." Compare sauce malapert "impertinence" (1520s), and slang phrase to have eaten sauce "be abusive" (1520s). Also compare salty in same senses.