E. Cobham Brewer 18101897. Dictionary of Phrase and Fable. 1898.
One who keeps a place warm for another, i.e. holds it temporarily for another. The allusion is to the custom in public schools of making a fag warm his superiors bed by lying in it till the proper occupant was ready to turn him out.
If Mr. Mellor took a judgeship, Grantham might object to become a warming-pan for ambitious lawyers.Newspaper paragraph, March 5th, 1886.