hush-hush vs undisclosed

hush-hush

adj

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

noun

  • secrecy; covert operation 

undisclosed

adj

  • Not disclosed; kept secret. 

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