To do something pointless, useless or difficult to achieve.
Exporting pine to Scandinavia is like carrying coal to Newcastle
Newcastle Upon Tyne in England was UK’s first coal exporting port and has been well-known as a coal mining centre since the Middle Ages, although much diminished in that regard in recent years. ‘Carrying coal to Newcastle’ was an archetypally pointless activity – there being plenty there already.
Other countries have similar expressions like in Spain “taking oranges to Valencia”.
In Germany you could hear «taking owls to Athens” (the owl signifies wisdom and Athenians are said to already have enough wisdom) and Italians say “selling snow to Eskimos”.
The last two expressions refer to things that are difficult to achieve, that is, requiring of superb sales skills, rather than being things that are pointless.
With increasing globalisation, parallels in other industries are being found, and the idiom is now frequently used by the media when reporting business ventures whose success may appear unlikely.
It has been referred to in coverage of British chicken tikka masala to India, the flowing of champagne and cheese from Britain to France, and the export of manga versions of William Shakespeare from Cambridge to Japan.
Amener du charbon à Newcastle
Correct French idiom
Porter de l’eau à la rivière