in vain vs thankless

in vain

adj
  • Unsuccessful, failed. 

adv
  • In a disrespectful manner, without honoring the proper meaning; insincerely or without proper respect. 

  • Without success or a result; ending in failure. 

thankless

adj
  • not appreciated or rewarded 

  • ungrateful or unappreciative 

How often have the words in vain and thankless occurred in a corpus of books? (source: Google Ngram Viewer )