Big Apple, the
New York City.
Origin of Big Apple, the
Dictionary.com Unabridged Based on the Random House Unabridged Dictionary, © Random House, Inc. 2019
the Big Apple informal New York City
Word Origin for Big Apple
C20: probably from US jazzmen's earlier use to mean any big, esp northern, city; of obscure origin
Collins English Dictionary - Complete & Unabridged 2012 Digital Edition © William Collins Sons & Co. Ltd. 1979, 1986 © HarperCollins Publishers 1998, 2000, 2003, 2005, 2006, 2007, 2009, 2012
"New York," 1909 (but popularized by 1970s tourism promotion campaign), apparently from jazz musicians' use of apple for any city, especially a Northern one.
Online Etymology Dictionary, © 2010 Douglas Harper