Wiktionary
n. (context idiomatic English) A pointless venture, in the sense of sending something to a place where it's made, or where they already have an abundance.
Wikipedia
Selling, carrying, bringing, or taking coal(s) to Newcastle is an idiom of British origin describing a foolhardy or pointless action. It refers to the fact that historically, the economy of Newcastle upon Tyne in north-eastern England was heavily dependent on the distribution and sale of coal—by the time of the first known recording of the phrase in 1538, 15,000 tonnes of coal were being exported annually from the area—and therefore any attempt to sell coal to Newcastle would be doomed to failure because of the economic principle of supply.
Usage examples of "coals to newcastle".
Adding a ruby ring to the blazing splendor of his costume was coals to Newcastle, but I was touched by the anxious thought behind it.
That show would have been coals to Newcastle at Athena, where practically everybody had been beaten regularly and severely when he was a little kid.