Definition of you so mean you so mean is informal of "you are so mean" which means they are rude or too direct with what they say or said.|"You so mean" is grammatical incomplete or incorrect. If you heard this spoken, you may have meant to write, "You're so mean," which is the contraction of "You are so mean." This sentence would be said to someone not being very nice. The "so" means "really ... More @Wikipedia
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