Green’s Dictionary of Slang

a place-holder, a deputy, used orig. of a clergyman who holds a living pro tempore.

1860		Hotten Dict. of Modern Sl. etc. (2nd edn) 248: A clergyman who holds a living pro tempore, under a bond of resignation, is styled a w.p., or warming pan rector, because he keeps the place warm for his successor. ? Clerical slang.
1873		Sl. Dict.