Knapsack

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early 17th century: from Middle Low German, from Dutch knapzack, probably from German knappen ‘to bite’ + zak ‘sack’.

wiktionary

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From Low German knapzak or Dutch knapzak (older form cnapsack), from Middle Dutch cnappen(“to bite with teeth”), ultimately from knappen(“to eat, crack”), of imitative origin, + sack. German Knappsack is from Dutch.

etymonline

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knapsack (n.)

c. 1600, from Low German Knapsack (16c.), probably from knappen "to eat" literally "to crack, snap" (imitative) + Sack "bag" (see sack (n.1)). Similar formation in Dutch knapzak.