biv·ou·ac (bĭv'ōō-āk', bĭv'wāk') n. A temporary encampment often in an unsheltered area. intr.v.
biv·ou·acked, biv·ou·ack·ing, biv·ou·acs also biv·ou·acks To camp in a bivouac.
[French, from German dialectal beiwacht, supplementary night watch : bei-, beside (from Middle High German bi-, from Old High German; see ambhi in Indo-European roots) + Wacht, watch, vigil (from Middle High German wahte, from Old High German wahta; see weg- in Indo-European roots).]
1702, from Fr., ult. from Swiss/Alsatian biwacht "night guard," from bei- "double, additional" + wacht "guard." Original meaning was an army that stayed up on night watch; sense of "outdoor camp" is 1853. Not a common word in Eng. before the Napoleonic Wars.