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 “superior’s” 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.