Forget (about) it!
1. Inf. Drop the subject!; Never mind!; Don't bother me with it. Jane: Then, there's this matter of the unpaid bills. Bill: Forget it! You'll have to pay them all! Sally: What's this I hear about you and Tom? Sue: Forget about it! I don't want to talk to you about it.
2. Inf. Nothing. Sue: What did you say? Mary: Forget it! Tom: Now I'm ready to go. Sue: Excuse me? Tom: Oh, nothing. Just forget it.
3. Inf. You're welcome.; It was nothing. John: Thank you so much for helping me! Bill: Oh, forget it!' Bob: We're all very grateful to you for coming into work today on your day off. Mary: Forget about it! No problem!