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.
kn-
knack
knacker
knackered
knap
knapsack
knapweed
knave
knavery
knavish
knead
