New York City hasn’t always been called the Big Apple. (For that matter, it hasn’t always been New York City either. What’s up, New Amsterdam?) But it does seem like a weird nickname for a metropolis that’s not particularly known for its orchards. So where did that nickname come from?
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In the 1920s, there was a reporter for the New York Morning Telegraph who covered horse racing, named John Fitz Gerald (sometimes spelled FitzGerald), as Barry Popik, Gerald Cohen and others have since noted. While there were several famous tracks in New York City, some of the best race horses came from the New Orleans area. Fitz Gerald heard stable hands referring to the races in New York, and their prestige and prize money, as the big apple of horse racing. Fitz Gerald found the term apt and started using it regularly in his columns on racing.
Source : https://history.howstuffworks.com/american-history/new-york-city-big-apple.htm