The word knapsack (sometimes spelled knapsacke in its early days) was first attested in a 1603 history of the Second Barons' War. It was borrowed either from the Dutch word knapzak or from Middle Low German Knapsack. Either way, it comes from two Germanic elements: one looking like cnappen or knappen and meaning "to eat" and the word sack, which meant "bag", just like the identical word we use today. Earlier on, cnappen meant "snap" or "crack" - the sound was thought to be similar to those made while eating - and that's ultimately of onomatopoeic origin. Sack comes from Proto-Germanic sakkuz, Latin saccus, and Proto-Germanic sakkos, all also meaning "bag". Finally, that might be of Semitic, because etymologists have identified similar words in Hebrew and Egyptian.
Hello! I'm Adam Aleksic, a sophomore studying government and linguistics at Harvard University, where I co-founded the Harvard Undergraduate Linguistics Society. In addition to etymology, I also really enjoy politics, vexillology, geography, board games, conlanging, and law.