What is the X in X-mas?
c.1400, "native sodium carbonate," from Old French nitre (13c.), from Latin nitrum, from Greek nitron, which is possibly of Eastern origin (cf. Hebrew nether "carbonate of soda;" Egyptian ntr). Originally a word for native soda, but also associated from Middle Ages with saltpeter (potassium nitrate) for obscure reasons; this became the predominant sense by late 16c.
(Prov. 25:20; R.V. marg., "soda"), properly "natron," a substance so called because, rising from the bottom of the Lake Natron in Egypt, it becomes dry and hard in the sun, and is the soda which effervesces when vinegar is poured on it. It is a carbonate of soda, not saltpetre, which the word generally denotes (Jer. 2:22; R.V. "lye").