Knapsack \Knap"sack`\, n. [D. knapzak; knappen to eat + zak a bag. See Knap, v. t., and Sack.]

A case of canvas or leather, for carrying on the back a soldier's necessaries, or the clothing, etc., of a traveler.

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

Source: Webster's Revised Unabridged Dictionary (1913)