bivouac vs bivouac sack

bivouac

noun
  • A temporary shelter constructed generally for a few nights. 

  • An encampment for the night, usually without tents or covering. 

  • Any temporary encampment. 

  • A structure formed by migratory ants out of their own bodies to protect the queen and larvae. 

verb
  • To encamp for the night without tents or covering. 

  • To set up camp. 

  • To watch at night or be on guard, as a whole army. 

bivouac sack

noun
  • A waterproof fabric shell designed to slip over a sleeping bag. 

How often have the words bivouac and bivouac sack occurred in a corpus of books? (source: Google Ngram Viewer )