weakyoudaoicibaDictYouDict[weak 词源字典]
weak: [13] Etymologically, something that is weak is ‘bendable’. The word was borrowed from Old Norse veikr. This was descended from prehistoric Germanic *waikwaz, which also produced German weich and Dutch week ‘soft’. And this in turn was formed from *waikw-, *wikw- ‘give way, yield’, a derivative of the base *wik- ‘bend’, which also produced the witch of English witch hazel [16] (etymologically the hazel with ‘bendy’ branches) and possibly English week.
[weak etymology, weak origin, 英语词源]
weak (adj.)youdaoicibaDictYouDict
c. 1300, from Old Norse veikr "weak," cognate with Old English wac "weak, pliant, soft," from Proto-Germanic *waikwa- "yield" (cognates: Old Saxon wek, Swedish vek, Middle Dutch weec, Dutch week "weak, soft, tender," Old High German weih "yielding, soft," German weich "soft"), from PIE root *weik- (4) "to bend, wind" (see vicarious).

Sense of "lacking authority" is first recorded early 15c.; that of "lacking moral strength" late 14c. In grammar, denoting a verb inflected by regular syllabic addition rather than by change of the radical vowel, from 1833. Related: Weakly. Weak-kneed "wanting in resolve" is from 1870.