German Rucksack pairs an Alpine word for the back with Sack. English borrowed the compound in the nineteenth century. Translate its parts and the apparently foreign word lands almost exactly on the familiar English backpack.
Sources
- Merriam-Webster · rucksack
Dictionary word history; the component and historical references below supply additional detail.
- Online Etymology Dictionary · rucksack
Historical forms, meanings, and context for the route described here.