Weak
Old English wāc ‘pliant’, ‘of little worth’, ‘not steadfast’, reinforced in Middle English by Old Norse veikr, from a Germanic base meaning ‘yield, give way’.
wiktionary
From Middle English weyk, wayk, weik, waik, from Old Norse veikr(“weak”), from Proto-Germanic *waikwaz(“weak, yielded, pliant, bendsome”), from Proto-Indo-European *weyk-(“to bend, wind”). Cognate with Old English wāc(“weak, bendsome”), Saterland Frisian wook(“soft, gentle, tender”), West Frisian weak(“soft”), Dutch week(“soft, weak”), German weich(“weak, soft”), Norwegian veik(“weak”), Swedish vek(“weak, pliant”), Icelandic veikur(“bendsome, weak”). Related to Old English wīcan(“to yield”). Doublet of week and wick. (Can this(+) etymology be sourced?)
This etymology is incomplete. You can help Wiktionary by elaborating on the origins of this term.
etymonline
weak (adj.)
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 from 1870.
