I could tell you but then I’d have to kill you

I could tell you, but then I'd have to kill you

humorous If I told you this extremely confidential or sensitive piece of information, I'd have to kill you to ensure that you don't share it with anyone else. Sure, I know who really stole the test answers, and I could tell you—but then I’d have to kill you. A: "I could tell you, but then I’d have to kill you." B: "Well then, I guess I'll never know the big secret, huh?"
See also: but, could, have, kill, tell
Farlex Dictionary of Idioms.

I could tell you but then I’d have to kill you

and ICTYBTIHTKY
sent. & comp. abb. a phrase said in answer to a question that one does not want to answer. Don’t ask. ICTYBTIHTKY.
See also: but, could, have, kill, tell
McGraw-Hill's Dictionary of American Slang and Colloquial Expressions
See also:
  • I could tell you, but then I'd have to kill you
  • ICTYBTIHTKY
  • mell
  • mell of a hess
  • that's my story and I'm sticking to it
  • (Don't ask me,) I just work here.
  • (Don't ask me,) I only work here.
  • Carborundum
  • illegitimi non carborundum
  • non illegitimi carborundum