"I could tell you, but then I'd have to kill you"--origin?

I thought so too.

I’m flashing back to a memory of one of the Summer Roadtrips with my father…

In the middle of a rather boring book on tape…
Me: “Hey Dad, do you know what Hotdogs are made of?”
Dad: “Well… Kosher hot dogs are made of pure beef, according to the package.”
Me: “And the rest?”
Dad: “I could tell you, but then you’d have to kill yourself.”