- The definition of a knapsack is a bag for carrying supplies that is worn on the back.
An example of a knapsack is a schoolbag.
A woman wearing a backpack or knapsack.Licensed from iStockPhoto
Origin: Du knapzak from knappen (see knap) plush zak, a sack
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Origin: Probably Low German Knappsack : knappen, to bite (probably of imitative origin) + Sack, bag (from Middle Low German sak, from Old High German sac, from Late Latin saccus; see sack1).