Apologies if this has been asked before, But how did the "Kiss" Michael gives to Fredo end up being so popular known as"the kiss of death" when there's no reference to that at all in the movie or the book?

The hero and the coward both feel the same thing, but the hero uses his fear, projects it onto his opponent, while the coward runs. It's the same thing, fear, but it's what you do with it that matters. Cus D'Amato