Origins
The word 'rucksack' entered English through one of the nineteenth century's most characteristically British pursuits: Alpine mountaineering. As British climbers pioneered routes in the Swiss, Austrian, and French Alps during the 1850s and 1860s — the so-called Golden Age of Alpinism — they adopted German-language equipment terminology alongside the sport. 'Rucksack,' the German word for the back-carried bag essential to any mountain expedition, was among the most natural of these borrowings.
The compound is formed from German 'Rücken' (back, the body part) and 'Sack' (bag, sack). In Swiss German and some Upper German dialects, 'Rücken' is contracted to 'Ruck,' giving the compound its familiar form. 'Rücken' descends from Old High German 'hrukki' (back, spine), from Proto-Germanic *hrugjō (back, spine, ridge). The English cognate is 'ridge' — from Old English 'hrycg' (back, spine, mountain ridge) — which preserves the metaphorical extension from the human back to the back of a mountain. The Scandinavian word 'rug' (as in a floor rug or blanket) also connects to this root, originally meaning a covering for the back.
'Sack' has one of the most widely traveled etymologies in the English language. German 'Sack' comes from Old High German 'sac,' borrowed from Latin 'saccus,' which came from Greek 'sákkos,' which was borrowed from a Semitic language — most likely Phoenician, related to Hebrew 'śaq' (sackcloth, a coarse fabric). The word traveled from the ancient Near East through Greek and Latin into virtually every European language: English 'sack,' French 'sac,' Italian 'sacco,' Spanish 'saco,' Russian 'мешок' (meshok, with different derivation but related borrowing patterns). It is one of the great wandering words of Mediterranean and European trade.
Germanic Development
The 'rucksack' is thus a compound with deep but divergent roots: the first element is purely Germanic, traceable to Proto-Germanic and possibly to PIE; the second is a Semitic word that entered European languages through ancient trade routes. This combination of Germanic and borrowed Semitic in a single compound is not unique — 'knapsack,' from Dutch 'knapzak' (bite-bag or snap-bag), combines a Germanic first element with the same Semitic-origin 'sack.'
In British English, 'rucksack' has remained the standard term for a back-carried bag, used in everyday as well as mountaineering contexts. In American English, the native compound 'backpack' (first attested in 1914) dominates ordinary usage, with 'rucksack' carrying connotations of European style, military equipment, or serious outdoor sport. The military usage is significant: the British Army has historically used 'rucksack' as its standard term, and the word entered American military slang through NATO cooperation and training exchanges.
The word 'ruck' — as in 'ruck march' (a military march with a loaded pack) — is a back-formation from 'rucksack,' clipping the compound to its first element. This usage is primarily American military, dating from the Vietnam War era. A 'ruck march' or 'rucking' has more recently entered civilian fitness culture as a form of weighted walking exercise.
Spelling and Pronunciation
Phonologically, English 'rucksack' preserves the German pronunciation quite closely, with the main adaptation being the English vowel quality: /ɹʌk.sæk/ versus German /ˈʁʊk.zak/. The German voiced /z/ in 'Sack' becomes English voiceless /s/, and the German /ʊ/ becomes English /ʌ/. The word is fully naturalized in English and requires no special marking or explanation.