unrelenting vs unthankful

unrelenting

adj
  • Having no pity; not being or becoming lenient, mild, gentle, or merciful 

  • Not relenting; not yielding in strength, severity, or determination. 

unthankful

adj
  • Not thankful; ungrateful. 

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