Not "X" is an interpretation.

Shouldn't the question be, "What exactly do you mean by 'not X'?" I think the question should be, "What exactly do you mean by 'not X'?

Example

Something's not working with the program!"

What exactly do you mean by "stuck?"

You're getting an error message when you compile.

Is the program you executed behaving differently than you expected?

What was the expectation and what was the actual behavior that occurred?

Mr. A has no communication skills!"

What exactly do you mean by "lack of communication skills?"

Did Mr. A not understand what the speaker said?

The speaker didn't understand what Mr. A told him?

What exactly are these "things we talked about"?

No masks."

This is pretty concrete and factual.

But I dare you to ask, "What exactly do you mean by 'no'?" Let's think about it.

Until last week, drugstores had plenty of masks, but now they don't."

So that mask no longer exists?

Not even at the factory?

It's at the factory, but it hasn't reached the store?

It's arriving in the store, but promptly sold out?

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