The Collaborative International Dictionary of English v.0.48:

Knapsack \Knap"sack`\, n. [D. knapzak; knappen to eat + zak a
   bag. See Knap, v. t., and Sack.]
   A case of canvas, leather, nylon, or other sturdy fabric,
   fitted with straps, for carrying on the back the food,
   clothing, or other supplies for a soldier or a traveler; as,
   to hike up the mountain with lunch in a knapsack.
   [1913 Webster +PJC]

         And each one fills his knapsack or his scrip
         With some rare thing that on the field is found.
                                                  --Drayton.
   [1913 Webster]

WordNet (r) 3.0 (2006):

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