I don't have the slightest idea.
→난 잘모르겠어.

〔예문〕
A: So how are you getting to the airport on Friday?
     그래 금요일에 공항에 어떻게 갈 거야?

B: I'm up in the air about that.
    아직 결정된 것 없어.

A: Are you going to take a taxi or a bus?
    택시타고 갈 거야 버스타고 갈 거야?

B: I don't have the slightest idea.
  난 잘 모르겠어.

* up in the air: 미결상태다 불확실하다
* (one) does not have the slightest idea: 전혀 모르겠다