noun
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.
兵士は背嚢を背負って行進した。
The soldiers marched carrying knapsacks on their backs.
古い登山用品店で背嚢を見つけた。
I found a knapsack at an old mountaineering shop.
Compound of 背 (hai, 'back') and 嚢 (nō, 'bag, pouch'). The kanji 嚢 is relatively rare in modern Japanese.