From wiki - believe if you dare
In the days of the Spanish New World Empire, the mainland of the American continent enclosing the Caribbean Sea and the Gulf of Mexico was referred to as the Spanish Main.
"Main" in this sense is a corruption of French and Anglo-French mesne, a reference to the fact that the Spanish Crown exercised its sovereignty over the land through various provincial governors and other intermediate landholders.
It included present-day Florida, the east shore of the Gulf of Mexico in Texas, Mexico, Central America and the north coast of South America. In particular, the term is most strongly associated with that stretch of the Caribbean coastline that runs from the ports of Porto Bello on the Isthmus of Darien, through Cartagena de Indias in New Granada, and Maracaibo to the Orinoco delta.
Veracruz in New Spain was another major port.
From the 16th to the early 19th century, the Spanish Main was the point of departure for enormous wealth that was shipped back to Spain in the form of gold, silver, gems, spices, hardwoods, hides and other riches.
Silver in the form of pieces of eight was brought to the Spanish Main by llama and mule train from Potosí via the Pacific coast, while wares from the Far East, that had arrived at Acapulco on the Manila Galleons, were also then transported overland to the Spanish Main.
From there they were shipped to Spain by Spanish treasure fleets. Due to the tremendous riches which were shipped from it, the Spanish Main was ripe territory for pirates and privateers.