The current American equivalent of the expression “”to cony coals to Newcastle” is “to sell refrigerators to the Eskimos.”
The idea is of doing something that is the height of superfluity.
In explanation, Newcastle, or Newcastle upon Tyne, to use the official name of this ancient English city, lies in the center of the great coal-mining region of England. Vast quantities of coal are shipped out of it by rail and by sea every day.
Hence, he would be a fool indeed who brought coal from another region into the place where it was naturally so plentiful.
The saying was recorded by Heywood in 1606; as he labeled it common even then, it may well go back a century or two earlier. Similar sayings occur in all languages.