In Dizzy Allen's book he says that a book called 'No Orchids for Miss Blandish' was very popular amongst the pilots on his squadron. The book was about gangsters and set in New York during Prohibition. The pilots though this a jolly wheeze and decided to copy the language. A newspaper journalist happened to be around and heard this. He thought it 'jolly good for morale' and published the story.
It may or may not be true: although I guess it's as good a reason as any.....