Definition from macmillandictionary.com
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‘Have they arrived yet?’ ‘I don’t know.’
How do you know my name?
I don’t know where the money’s coming from.
I don’t know anything about it.
Some of you may know Ivan already.
used informally, in spoken language
used when you have no information and you are unable to guess the answer to a question
used when you do not know something and you feel annoyed that someone is asking you about it
It beats me
used for saying that you do not know or understand something
used for saying that you don’t know something because it is impossible for anyone to know it
used for saying that you know as little about something as the person who asked you about it
used for saying that something may be true, but you do not have enough information to know whether it is or not