forget it

It doesn't matter.

“What were you saying? – Oh, it was nothing important, forget it.”

Used a conventional reply to an expression of gratitude.

“Thank you so much! How can I repay you for your kindness? – Forget it; it was nothing.”

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

“It's raining. I said, "It's raining." Do you hear me? It's raining! Oh, forget it!”