Origins
The English adjective 'weak' has a layered history that involves both borrowing and replacement within the same language family. It entered Middle English around 1300 from Old Norse 'veikr,' meaning 'pliant, flexible, yielding, weak.' The Old Norse word descended from Proto-Germanic *waikwaz, from the PIE root *weik-, meaning 'to bend, to wind, to turn.' At its etymological core, weakness is pliability — the quality of bending under pressure, of yielding rather than holding firm.
This borrowing from Old Norse is part of the enormous Scandinavian influence on English during the Danelaw period (ninth to eleventh centuries), when large areas of northern and eastern England were settled by Norse-speaking Vikings. What makes the story of 'weak' unusual is that English already had a native word from the same Proto-Germanic root: Old English 'wāc' (soft, pliant, weak, feeble). The Norse form 'veikr' replaced the native 'wāc' — a case of one Germanic cousin displacing another. The Norse form won out perhaps because its vowel was more distinctive, or because Norse prestige in the Danelaw was sufficient to favor Norse vocabulary even when English had a near-equivalent.
The PIE root *weik- (to bend) produced a wider family of English words than is immediately apparent. 'Wicker' (pliable woven twigs) preserves the concrete sense of bending: wicker is material that can be bent and woven. 'Wych' or 'witch' in the compound 'wych elm' (a species of elm with pliant branches) comes from the same root — the wych elm is the bending elm, named for its flexible wood. The Latin development of the same PIE root produced 'vīcis' (change, alternation — literally a turning), which entered English in the compound 'vicissitude,' and 'vicia' (vetch, a climbing plant that winds around supports).
Proto-Indo-European Roots
The Proto-Germanic cognates show the typical pattern of related but diverging meanings. German 'weich' means 'soft' (not weak) — soft bread, a soft pillow, a soft voice. Dutch 'week' means both 'soft' and 'weak.' Swedish 'vek' means 'soft, tender.' The divergence illustrates a common pattern: Proto-Germanic *waikwaz meant 'yielding, pliant,' and different daughter languages specialized this in different directions — toward physical softness (German), toward lack of strength (English), or toward emotional tenderness (Swedish).
The grammatical term 'weak verb' was coined by Jacob Grimm as the counterpart to 'strong verb.' Weak verbs form their past tense by adding a dental suffix ('-ed' in English: walk/walked, talk/talked), relying on external addition rather than internal vowel change. Grimm considered this 'weaker' than the 'strong' internal alternation of verbs like sing/sang/sung. The terminology is conventional rather than evaluative — weak verbs are actually the productive, growing class in English, while strong verbs are a closed, shrinking set.
In Old English, the native adjective 'wāc' appears in poetry and prose with meanings ranging from 'soft' to 'cowardly' to 'insignificant.' The compound 'wācmōd' (weak-spirited, faint-hearted) shows that the metaphorical extension from physical pliability to psychological feebleness was already established in the pre-Norse period. When 'weak' from Norse replaced 'wāc,' it inherited and continued these metaphorical uses.
Middle English
The noun 'weakness' and the verb 'weaken' are both Middle English formations using standard English suffixes. 'Weakling,' meaning a person or animal of feeble constitution, appeared in the sixteenth century. The compound 'weak-kneed' (literally having knees that buckle, hence cowardly or irresolute) is attested from the mid-nineteenth century.
Modern English 'weak' has developed a wide range of specialized uses. In grammar, a weak form is an unstressed pronunciation of a function word (the weak form of 'can' is /kən/ rather than /kæn/). In chemistry, a weak acid is one that does not fully dissociate in solution. In phonetics, a weak syllable is unstressed. In card games, a weak hand is one with few high cards. In economics, a weak currency is one declining in value. All these specialized uses preserve the core metaphor of insufficiency — of something that does not fully hold, that yields under pressure.
The opposition between 'strong' and 'weak' is etymologically elegant. 'Strong' comes from PIE *strenk- (tight, taut), and 'weak' comes from PIE *weik- (to bend). The pair maps perfectly onto a physical image: the strong thing is pulled tight and rigid, the weak thing is pliant and bending. This is not coincidence but rather the reflection of a deep, cross-linguistic tendency to conceptualize strength and weakness in terms of physical tension and flexibility. The same opposition appears in Latin 'fortis' (strong, from *bʰerǵʰ- 'high, strong') versus 'mollis' (soft, yielding), and in many other language families.