What is the X in X-mas?
c.1300, "off to one side;" mid-14c., "to or from the side;" late 14c., "away or apart from others, out of the way," from a- (1) + side (n.). Noun sense of "words spoken so as to be (supposed) inaudible" is from 1727. Middle English had asidely "on the side, indirectly" (early 15c.) and asideward "sideways, horizontal" (late 14c.).