1702, "encampment of soldiers that stays up on night watch in the open air, dressed and armed," from French bivouac (17c.), said to be a word from the Thirty Years' War, ultimately from Swiss/Alsatian biwacht "night guard," from bei- (from Old High German bi- "by," here perhaps as an intensive prefix; see by) + wacht "guard" (from Proto-Germanic *wahtwo, from PIE root *weg- "to be strong, be lively"). Sense of "outdoor camp" is from 1853. According to OED 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 without tents" is from 1814.