rucksack
Synonym discussion
Sense: noun/1
These three words all refer to bags worn on the back, but they differ in usage and connotation. 'Backpack' is by far the most common word, used for school bags, travel bags, and hiking bags alike. 'Rucksack' — borrowed from German in the 19th century — is more common in British English and carries a stronger association with hiking, camping, and outdoor adventure; it often suggests a sturdier, more technical bag. 'Knapsack' is the oldest of the three, dating from the 17th century, and today sounds old-fashioned or military. A knapsack typically implies a smaller, simpler bag without a metal frame. For everyday use, 'backpack' is the safest choice; if you are writing about mountain trekking for a British audience, 'rucksack' sounds natural.
Common collocations
Etymology
From German 'Rucksack', formed from 'Rücken' (back) and 'Sack' (bag). The word entered English in the mid-19th century, originally used by British soldiers and mountaineers who borrowed the term from German and Austrian hiking culture.