The Collaborative International Dictionary of English v.0.48:

Haversack \Hav"er*sack\ (h[a^]v"[~e]r*s[a^]k), n. [F. havresac,
   G. habersack, sack for oats. See 2d Haver, and Sack a
   bag.]
   [1913 Webster]
   1. A bag for oats or oatmeal. [Prov. Eng.]
      [1913 Webster]

   2. A bag or case, usually of stout cloth, in which a soldier
      carries his rations when on a march; -- distinguished from
      knapsack.
      [1913 Webster]

   3. A gunner's case or bag used to carry cartridges from the
      ammunition chest to the piece in loading.
      [1913 Webster]

WordNet (r) 3.0 (2006):

haversack
    n 1: a bag carried by a strap on your back or shoulder [syn:
         backpack, back pack, knapsack, packsack,
         rucksack, haversack]