Wikipedia
A bed warmer was a common household item in countries with cold winters, especially in Europe. It consisted of a metal container, usually fitted with a handle and shaped somewhat like a modern frying pan, with a solid or finely perforated lid. The pan would be filled with hot coals and placed under the covers of a bed, to warm it up and/or dry it out before use.
After the invention of rubber, the classical bed warmer was largely supplanted by the hot water bottle, which is still widely used. In the early 20th century, electric blankets and then the electric bed warmer, (containing a lampholder and low wattage light bulb), were invented to fulfill the same need.
Usage examples of "bed warmer".
If the room was cold, one could always warm up in the Great Hall before retiring, send a servant in with a bed warmer first, and then bury oneself in blankets with a hot brick for comfort at bedtime.
She was buried under a thick layer of coverlets, the sheets made comfortable when one of the maids had passed a bed warmer between them earlier.
It was a pity Rosalie wasn't with him, buthe looked over at his companionMandy was an agreeable bed warmer, if a little too skinny for his usual tastes.