What it means “all you”?
What it means when someone says “all you” after you compliment them?

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You need to have a preposition for that. Like "all of you" 
But if you consider the meaning of Everybody or people in general, It means all people (when you call them, and they are considered as the object of the verb)

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May be you misunderstood and the response was "And you."
Like this exchange;

A: It has been a pleasure to meet you.
B: And you.

This is a way of saying; Yes, I feel the same way.

A lady wearing a fancy dress is meeting her friend. The friend will be attending the same party and is also wearing a fancy dress. 
A: You look lovely this evening.
B: And you.

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I commented here by mistake. So I deleted the text here. Sorry.

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Who knows. I'd like to know too.

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Sorry Mia, that doesn't mean anything in English.  Perhaps you misheard them?