c.1300, from Old Norse veikr "weak," cognate with Old English wac "weak, pliant, soft," from Proto-Germanic *waikwaz "yield," *wikanan "bend" (cf. 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- "to bend, wind" (see vicarious). Sense of "lacking authority" is first recorded early 15c.; that of "lacking moral strength" late 14c. Weak-kneed "wanting in resolve" is from 1870.