know (someone or something) through and through
To have a thorough and complete knowledge of someone or something. I know Elizabeth through and through, so I really doubt that she's hiding something. A: "You can recite that poem from memory?" B: "Sure, I know it through and through."
know something through and through
to know something very well. I want you to know this project through and through before the staff meeting. I know my part in the play through and through.