What does the word bivouac mean, I think it's military, and how did it come about?

Bivouac.

"A bivouac (pronounced /ˈbɪvuːæk/, BIV-oo-ak) traditionally refers to a military encampment made with tents or improvised shelters, usually without shelter or protection from enemy fire or such a site where a camp may be built. It is also commonly used to describe a variety of improvised camp sites such as those used in scouting and mountain climbing. It may often refer to sleeping in the open with a bivouac sack, but it may also refer to a shelter constructed of natural materials like a structure of branches to form frame may be utilized, which is then covered with leaves, ferns and similar for waterproofing and duff (also known as leaflitter) for insulation.

As a verb, to bivouac (alternatively bivouacked, bivouacking, bivouacs also bivouacks) is to set up or camp in any such improvised camp....The term bivouac was introduced into English from French but did not become common until after the Napoleonic Wars; it ultimately stems from the Alsatian or Swiss German word biwacht. (Broken down, it reads bie- (the modern German equivalent is bei-) for subordinate, secondary and wacht for guarding as in secondary night watch.)"

https://en.wikipedia.org/wiki/Bivouac_shelter