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Its an abbreviation of bivouac.

bivouac
ˈbɪvʊak,ˈbɪvwak noun

  1. a temporary camp without tents or cover, used especially by soldiers or mountaineers.
    verb
  2. stay in a bivouac.
    "we bivouacked on the north side of the town"

bivvy
ˈbɪvi/informal noun
noun: bivvy; plural noun: bivvies

  1. a small tent or temporary shelter. verb
    verb: bivvy; 3rd person present: bivvies; past tense: bivvied; past participle: bivvied; gerund or present participle: bivvying
  2. stay in a small tent or temporary shelter.
    "he opted to bivvy up for two days"

My bad. That's what assuming does :)

That's just so funny, i couldn't resist ...sorry..... LOL ;)