forget it vs say again

forget it

verb
  • Don't let it bother you; put it out of your mind. 

  • I don't want to continue with this conversation (usually used in frustration or anger). 

  • It doesn't matter. 

  • A conventional reply to an expression of gratitude. 

  • Absolutely not; I forbid it or it is impossible. 

say again

phrase
  • To repeat part of or entire message transmitted over the radio, used instead of "repeat" which is an artillery procedure word. 

  • "What did you say?" or "Repeat what you have said." A polite formula used when one has not heard or understood what has been said. 

How often have the words forget it and say again occurred in a corpus of books? (source: Google Ngram Viewer )