I don't know

Short definition from Wiktionary

Interjection

1. 1
   Used in response to a question or command in which the target person does not know the answer to or how to respond.
2. 2
   Used to indicate that one is speculating (often used sarcastically when stating the obvious).
3. 3
   Used to express exasperation or incredulity about a given situation or person.
4. 4
   Used other than figuratively or idiomatically: see I, don't, know.