From Longman Dictionary of Contemporary EnglishI don’t knowI don’t knowa) NOT KNOWused to say that you do not have the answer to a question ‘When did they arrive?’ ‘I don’t know.’ b) SUREused when you are not sure about something ‘How old do you think he is?’ ‘Oh, I don’t know – sixty, seventy?’I don’t know what/how/whether etc I don’t know whether to call him.I don’t know that I don’t know that you need a passport for travelling within the EU. c) DISAGREEused to show that you disagree slightly with what has just been said ‘I couldn’t live there.’ ‘Oh, I don’t know. It might not be so bad.’ d) ANNOY British English used to show that you are slightly annoyed Oh, I don’t know! You’re hopeless! → know
Examples from the Corpus
I don’t know• "Where's the nearest restroom?" "I don't know."• "I couldn't live there.'' "Oh, I don't know. It might not be so bad.''• Oh, I don't know, we could meet at 3 or 4 o'clock. 