hush-hush vs stealthiness

hush-hush

noun
  • secrecy; covert operation 

adj
  • Secret, not spoken of (or spoken of only in hushed whispers between insiders). 

stealthiness

noun
  • The property of being stealthy. 

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