资料来源 : pyDict

膝盖弱的,软弱的,优柔寡断的

资料来源 : Webster's Revised Unabridged Dictionary (1913)

Weak-kneed \Weak"-kneed`\, a.
   Having weak knees; hence, easily yielding; wanting
   resolution. --H. James.

资料来源 : WordNet®

weak-kneed
     adj : lacking will power or resolution; "the role of the dissenter
           is not for the weak-kneed"