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Phrase(s): Don’t I know you from somewhere?
a way of striking up a conversation with a stranger, as at a party or other gathering. • Bill: Don’t I know you from somewhere? Mary: I don’t think so. Where did you go to school? • Henry: Don’t I know you from somewhere? Alice: No, and let’s keep it that way.
McGrawhill's American Idioms And Phrasal Verbs