There are many theories as to the origin of the expression, 'The Real McCoy'.
One contender is that it refers to the Canadian inventor Elijah McCoy, born in
or around 1844, who invented many things including an automatic lubricator for
steam engines. Other automatic lubricators followed but were considered inferior thus engineers were said to ask for "the real McCoy".
Another is that it originates in Edinburgh, Scotland where, by 1870, the
"the Real Mackay" was the established advertising slogan of the G. Mackay and Co. whisky distillery company whose whiskey was exported to both the USA and
Canada. The Oxford English Dictionary records a letter written by Robert Louis Stevenson in 1883 referring to somebody as 'the real Mackay'.
Another is that it originates with legendary boxer Kid McCoy (1873-1940) who was
so popular that many fighters assumed the same name. To reassure their readers that they were reporting on the genuine article, an 1899 edition of the San Francisco Examiner stated that the "real McCoy" had won his latest match.
Yet another theory comes from a member of the McCoy family of Long Island, NY, who says that his great grandfather ran a funeral parlor in Lower Manhattan during the prohibition era. Few corpses passed through the door because the establishment was primarily a front for a whiskey distillery which supplied the local speakeasies, who would ask for Real McCoy Whiskey.
While it appears that the oldest known variation of this expression originates in Scotland, it also seems quite conceivable that it was moderated into its modern-day usage by one or more of the other 'originators' - perhaps as a pun on the original.
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