The Collaborative International Dictionary of English v.0.48:

Bivouac \Biv"ouac\, v. i. [imp. & p. p. Bivouacked (?); p. pr.
   & vb. n. Bivouacking.] (Mil.)
   (a) To watch at night or be on guard, as a whole army.
   (b) To encamp for the night without tents or covering.
       [1913 Webster]