What is the X in X-mas?
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." Cf. sauce malapert "impertinence" (1520s), and slang phrase to have eaten sauce "be abusive" (1520s). Also cf. salty in same senses.