flatly vs undoubtably

flatly

adv
  • In a definite manner; in a manner showing complete certainty. 

  • In a physically flat or level manner. 

  • In a manner that shows no emotion. 

undoubtably

adv
  • Without doubt; indubitably, undoubtedly. 

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