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
