forget it

Short definition from Wiktionary

Phrase

1. 1
   Absolutely not; I forbid it or it is impossible.
2. 2
   It doesn't matter.
3. 3
   A conventional reply to an expression of gratitude.
4. 4
   I don't want to continue with this conversation (usually used in frustration or anger).
5. 5
   Don't let it bother you; put it out of your mind.