headstrong vs weak-kneed

headstrong

adj
  • Determined to do as one pleases, and not as others want. 

weak-kneed

adj
  • Used other than figuratively or idiomatically: see weak, kneed. 

  • Lacking will power or strength of character; timid. 

How often have the words headstrong and weak-kneed occurred in a corpus of books? (source: Google Ngram Viewer )