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A lazy way of telling someone to shut up because you're too tired to listen to their whining and you're not going to help them no matter what they say.
My mom said, "I don't know what to tell you" after I cried for an hour about my math test.
My friend said, "I don't know what to tell you" when I told him I failed my history quiz.
My teacher said, "I don't know what to tell you" after I asked for extra credit for the 10th time.