An expression used to call someone on an obvious lie, prevarication or falsehood. A way of calling bullshit. Since it is now known Elvis did mountains of drugs, stating that he had always been clean would be obvious bullshit. Origin: Penn and Teller's show Bullshit, where the phrase is used to call bullshit on the various claims of creationists.
Mike: "Elton John invented rap music."
Bob: "Um, right, Elvis didn't do no drugs!"