bivouacyoudaoicibaDictYouDict[bivouac 词源字典]
bivouac: [18] Bivouac appears to be of Swiss- German origin. The early 19th-century writer Stalder noted that the term beiwacht (bei ‘additional’ + wacht ‘guard’ – a relative of English watch and wake) was used in Aargau and Zürich for a sort of band of vigilantes who assisted the regular town guard. Beiwacht was borrowed into French as bivac, and came to English in a later form bivouac.

Its original application in English was to an army remaining on the alert during the night, to guard against surprise attack; in so doing, of course, the soldiers did not go to sleep in their tents, and from this the term bivouac spread to ‘improvised, temporary camp’, without the luxury of regular tents.
=> wake, watch[bivouac etymology, bivouac origin, 英语词源]
bivouac (n.)youdaoicibaDictYouDict
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.