bivouac [E18th] ‘A night watch by the whole army’ was the original meaning of bivouac. The origin is French, probably from Swiss German Bîwacht ‘additional guard at night’, apparently referring to a citizens’ patrol giving support to the ordinary town watch. The word is said to have been introduced into English during the Thirty Years War (1618–48). The abbreviation bivvy is recorded from the early 20th century, apparently among Anzac troops (see australian english).