Brewer's: Sylvam Lignum Ferre
(In). To carry coals to Newcastle. The French say, “ Porter de l'eau à la rivière. ” To do a work of supererogation; to paint the lily, or add another perfume to the violet, or perform any other superfluous or ridiculous excess.
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894