Who does "you" refer to here? What kind of expression is it?
The word "you" is actually not the point of emphasis here, it is "I could not tell you." This expression is used when someone doesn't know the answer to a question. It has a similar meaning to "I don't know" or "I have no idea," but may take on a hint of sarcasm or incredulity depending on the speaker's tone. Ex. A: Is it going to rain tomorrow? B: I couldn't tell you.
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