Noir Mystery: What's the Meaning of the Title, "The Postman Always Rings Twice"

It’s the title of the great James M. Cain novel, the great John Garfield/Lana Turner fim noir, and the sexy Jessica Lange/Jack Nicholson re-make.

It’s a geat novel. Cain was a truly great American writer and I’ve re read it often. But every time I’ve re read it, I’ve found myself wondering about the title. I don’t remember coming across the phrase “the postman always rings twice”, in the novel, and I don’t think it is heard in either film. I’d always assumed that it was the punch line to a vintage dirty joke, probably about an adulterous housewife (which in a murderous way the Cain plot was about). Something like the really bad dirty joke about the way a “Chinaman” makes love in Chinatown. In fact I thought that deliberately bad dirty joke was meant as an homage to the kind of joke that gave Cain the title for “Postman”.

But what does it mean? What is the lost joke? I’ve asked a lot of people over the years and no one seems to know. Some are not even certain it’s the punchline to a joke. That it stands alone as some comment on the insistent inexorability of fate. The double meaning of identity. (interesting that the doubling re apears in Cain’s next novel Double Indemnity)

But I think it sounds like a punchline and I want to know the joke. Does anybody know the answer?