not have the foggiest/faintest (idea)完全不知；一头雾水(类似have no idea)。

例：- Where are we? 我们在哪？- I'm afraid I haven't the foggiest. 再如：The photocopier's broken down again, and nobody has the foggiest idea how to fix it. 复印机又坏了，但我们对修理一窍不通。