doubtless vs undoubtably

doubtless

adv
  • Without doubt; very probably, in all likelihood; doubtlessly. 

adj
  • Characterized by or experiencing no doubt at all, certain; undoubted; undoubting. 

undoubtably

adv
  • Without doubt; indubitably, undoubtedly. 

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