I don't know

exclamation3

1. •
   Used in response to a question or command in which the target person does not know the answer to or how to respond.

   • Mr. Schidtler: Does anybody here have any idea why "Inestine" was regarded as a great scientist? Kevin? Kevin: Oh, uh, I don't know. — 1982
2. •
   Used to indicate that one is speculating (often used sarcastically when stating the obvious).

   • Maybe you could, I don't know, directly talk to her about it instead of complaining to me for once?
3. •
   (UK) Used to express exasperation or incredulity about a given situation or person.

   • The audacity of that lad... I don't know.