nay vs not really

nay

adv
  • Or rather, or should I say; moreover (introducing a stronger and more appropriate expression than the preceding one). 

noun
  • A vote against. 

  • A person who voted against. 

not really

adv
  • Not actually (a hypothetical will not, in fact, come to pass), often rhetorical. 

  • To a low degree, not particularly, not especially. 

How often have the words nay and not really occurred in a corpus of books? (source: Google Ngram Viewer )