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Phrase(s): What do you know for sure?
Inf. How are you?; What do you know? (Familiar. An elaboration of What do you know? Does not require a direct answer.) • Tom: Hey, man! What do you know for sure? Bill: Howdy, Tom. What’s new? • John: How are you doing, old buddy? Bill: Great, John! John: What do you know for sure? Bill: Nothing much.
McGrawhill's American Idioms And Phrasal Verbs