The phrase to carry coals to Newcastle means to supply something to a place where it is already plentiful; hence, figuratively, to do something wholly superfluous or unnecessary—cf. also to sell refrigerators to the Eskimos and to sell sand in the Sahara.

This phrase (in which coals is an obsolete plural) refers to Newcastle upon Tyne, in Northumberland (north-eastern England). According to the Encyclopædia Britannica, when the expression came into being in the 16th century, locally mined coal destined for the growing London market was Newcastle’s principal export, so that no one needed to take coal there.

The phrase is first recorded in 1583 in the diary of the Scottish theologian and reformer James Melvill (1556-1614). As a rendering of Greek γλαύκ είς Άθήνας (owls to Athens), he used “salt to Dysert, or colles to Newcastell” (Dysart, in Fife, Scotland, was an important salt port).

The English playwright Thomas Heywood (circa 1573-1641) used coals from Newcastle as the image of ordinariness in the second part of If you know not me, you know no bodie: Or, The troubles of Queene Elizabeth (1605). At the beginning of the play, Hobson, a haberdasher, is talking to a “Pedler with tawnie coate”:

– Hobson: What newes i’th’ country? what commodities
Are most respected with your Country Girls?
[…]
– Pedler: Many of our young married men, haue tane an order to weare yellow garters, points, and shootyings; and tis thought yellow will grow a custome.
– Hobson: ’Tas been vs’de long at London.
– Pedler: And tis thought ’twill come in request in the Country, too: for a fashion that three or four young wenches have promised mee their husbands shall weare, or theyle misse of their markes. Then your maske, silke-lace, washt gloues, carnation girdles, and bulk-point sutable, as common as coales from Newcastle: you shall not haue a kitchin-maid scrape trenchers without her washt gloues; a darie-wench will not ride to market, to sell her butter-milke, without her maske and her buske.

The phrase was later used in The history of the worthies of England who for parts and learning have been eminent in the several counties: together with an historical narrative of the native commodities and rarities in each county (1662), by the English churchman and historian Thomas Fuller (1608-61). In the section Proverbs of the worthies of Northumberland, Fuller wrote:

To carry Coals to Newcastle.
That is to do, what was done before, or to busy ones self in a needless imployment. Parallel to the Latine, Aquam mari infundere [to pour water into the sea], Sidera Coelo addere [to add stars to the sky], Noctuas Athenas; To carry Owles to Athenes, which place was plentifully furnished before with fowle of that feather.

(The French equivalent, now obsolete, is porter de l’eau à la rivière, to carry water to the river.)

So common was the phrase at that time that Sir Robert Atkins said the following in the House of Lords on 17th December 1666 during the Report of the Conference on the Bill to prohibit the Importation of Irish Cattle:

(originally published by His Majesty’s Stationery Office, London, 1767-1830)
We have Cattle enough of our own; and the multiplying of those beyond Proportion may be a Mischief, and may be as sending Coals to Newcastle,  which would be little welcome there; for that will occasion the leaving our Grounds unstocked and unmanured; it drains us of our Money, and supplies us with that we need not.