What is the X in X-mas?
1702, from French bivouac (17c.), ultimately from Swiss/Alsatian biwacht "night guard," from bei- "double, additional" + wacht "guard" (see wait (v.)). Original meaning was an army that stayed up on night watch; sense of "outdoor camp" is 1853. Not a common word in English before the Napoleonic Wars. Italian bivacco is from French. As a verb, 1809, "to post troops in the night;" meaning "camp out of doors" is from 1814.