forget it vs not at all

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. 

not at all

intj
  • Used similarly to you're welcome, as a conventional reply to an expression of gratitude. 

  • Don't worry about it (response to worry or an apology) 

adv
  • Not in the least. 

How often have the words forget it and not at all occurred in a corpus of books? (source: Google Ngram Viewer )