Carry Coals To Newcastle Meaning

Example : 1851, Herman Melville, Moby-Dick, Chapter 81:  However curious it may seem for an oil-ship to be borrowing oil on the whale-ground, and however much it may invertedly contradict the old proverb about carrying coals to Newcastle, yet sometimes such a thing really happens; and in the present case Captain Derick De Deer did indubitably conduct a lamp-feeder as Flask did declare.1960, P. G. Wodehouse, Jeeves in the Offing, chapter II:  He's the fellow who likes to let off stink bombs in night clubs, which rather falls under the head of carrying coals to Newcastle [...]