(informal, C2) used to emphasize that you do not know something

"Is she going to stay?" - "I haven't have the faintest idea."

I don't have the faintest idea what you're talking about.

Neither of them had the faintest idea of how to be a parent.

As to what I'm going to do now, I haven't the faintest idea.

I don't think Steve had the faintest idea about what was happening around him.