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Phrase(s): Don’t you know it!
You can be absolutely sure about that!; You’re exactly right, and I agree with you. (This is not a question.) • Alice: Man, is it hot! Fred: Don’t you know it! • Bob: This is the best cake I have ever eaten. The cook is the best in the world! Bill: Don’t you know it!
McGrawhill's American Idioms And Phrasal Verbs