— selling coals to Newcastle (PHRASES) To do something superfluous; something supplied to a place where it is already plentiful
— ORIGIN Old English.
Newcastle is a port in the North East of England on the River Tyne. It was once the center of all UK coal exports, having a healthy coal seam. Hence, it would be stupid for some one to try and sell coal to Newcastle.
The French have a similar saying "Porter de l'eau à la riviere" (to carry water to the river). There are numerous Latin equivalents as, "To carry wood to the forests" ("Poma Alcinoo dare") etc.
So basically selling coals to Newcastle is to do something unnecessary or superfluous.
"Taking my boyfriend to the latest Orlando Bloom film would be like taking coals to Newcastle"
"To carry or take coals to Newcastle"