A basic, vague, or rudimentary knowledge or understanding (about something). Often used in negative constructions. A: "Do you know where the car keys are?" B: "I haven't the foggiest. Sorry." Do you have the foggiest how much this mistake is going to cost the company? Even though I could speak a little Japanese, I could only pretend to have the foggiest what the woman was talking about.

Knowledge or understanding of something. Typically used in the negative to show that one does not know something. He does not have the foggiest idea how hard it is to run a business. Where are your keys? I haven't the foggiest.