A loanword from German 'Sack', used in Japanese for a rucksack or backpack, often in outdoor or casual contexts. Compare with リュックサック, which is more common.
from German Sack
See also: リュックサック
山登りにはザックが欠かせない。
A rucksack is essential for mountain climbing.
I climbed the mountain trail carrying a heavy rucksack on my back.
出発前に食料と水をザックに詰めた。
Before setting off, I packed food and water into my rucksack.
From German 'Sack', meaning sack or bag.