Rucksack comes from German Rucksack, from dialectal Rucken (standard Rücken, 'back') + Sack ('bag, sack'), thus literally 'back-bag'. The historical chain includes German Rucksack. The quoted older senses, "back", "bag, sack", and "back-bag", supply the semantic anchors for rucksack. The formation is best read as compound and borrowed: the word rucksack carries its older form-history into a compact English lexical shape. In present English it is used as a noun, "a bag with shoulder straps carried on the back; a backpack". The result is a noun whose etymology preserves both the older form and the later English specialization.