also *weig-, Proto-Indo-European root meaning "to bend, to wind."
It forms all or part of: vetch; vicar; vicarious; vice- "deputy, assistant, substitute;" viceregent; vice versa; vicissitude; weak; weakfish; week; wicker; wicket; witch hazel; wych.
It is the hypothetical source of/evidence for its existence is provided by: Sanskrit visti "changing, changeable;" Old English wac "weak, pliant, soft," wician "to give way, yield," wice "wych elm," Old Norse vikja "to bend, turn," Swedish viker "willow twig, wand," German wechsel "change."
c. 1300, from Old Norse veikr "weak," cognate with Old English wac "weak, pliant, soft," from Proto-Germanic *waika- "yield" (source also of 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- (2) "to bend, to wind."
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 by 1856; older in a literal sense.