The phrase, "carrying coals to Newcastle," means spending an inordinate amount of energy on something useless, fruitless, or redundant. This idiom arose in the 15th century because Newcastle, England was known throughout the country as a major exporter of coal. Therefore, "carrying coals to Newcastle" would do you no good, because there was more coal there than anywhere else. Variations on the saying include "bringing," "taking," or "moving" the coal.
Newcastle-on-Tyne was founded at a convenient place to cross the Tyne River in 1080 as a harbor town. During the Middle Ages, the city exported wool by boat down the river. Then, in the 1400s, Newcastle became famous for its abundant coalmines. They exported the fuel all over England, especially to the growing metropolis of London. At the time, "coals" referred to the lumps of raw coal, whereas in today's vocabulary, "coal" is plural.
The surrounding counties of Northumberland and Durham supported a biannual fair in Newcastle where peddlers sold their goods. Perhaps, "carrying coals to Newcastle" arose as advice between peddlers not to try to sell coal at the bazaar. The first recorded instance of the contextualized saying appears in 1538 in England.
Idioms are creative sayings that use examples or turns of phrase that are specific to a language or culture. However, the spirit and meaning of idioms are frequently universal. For example, both the Dutch and Spanish having sayings, "like bringing water to the ocean." In Poland and Sweden, you'd hear, "bringing wood to the forest." Some regionally specific idioms for redundancy include Russia's "taking samovars to Tulu," a city famous for its spigotted teapots. The Greek spread the saying, "bringing owls to Athens," since the night predator is a symbol of the ancient city named after the goddess Athena. Of course, in the United States we have our own version: It's often said that bringing a date to a wedding is "like bringing sand to the beach."
Using idioms correctly improves your cultural literacy. One frequently encounters, "carrying coals to Newcastle" in newspapers and magazines. Ironically, in 2004 Newcastle began importing coal from Russia. Despite the phrase no longer holding literal truth, the well-known idiom will certainly live on.