A bag carried on the back, often used in military or hiking contexts. The term is somewhat dated; modern Japanese more commonly uses リュックサック or バックパック.
The soldiers marched carrying knapsacks on their backs.
I found a knapsack at an old mountaineering shop.
Standard kanji form for 'knapsack'.
Search-only kanji form; not a standard written form.
Compound of 背 (hai, 'back') and 嚢 (nō, 'bag, pouch'). The kanji 嚢 is relatively rare in modern Japanese.