IDIOMS

weak-kneed

(literally) Used other than as an idiom: see week,‎ kneed.

Example: 1903, Jack London, The People of the Abyss, ch. 5:
  It is incontrovertible that the children grow up into rotten adults, without virility or stamina, a weak-kneed, narrow-chested, listless breed, that crumples up and goes down in the brute struggle for life.