Before the days of
electric blankets and
central heating it was very difficult for people to not only get their bed warm but keep it that way in order to achieve a restful slumber. Sometimes blankets just weren't enough. So ingenious devices were devised in order to bring the
fireplace to the bed itself, without setting the entire house on fire.
There were many different ways to accomplish this. Predominantly, a metallic container was connected to a long stick and put into the fireplace. There, burning hot coals from the fire would be captured, a top would be placed upon it, and then the container would be slid above the mattress and below the bed covers in order to warm up the bed. This was often one of many chores for the maid during the winter months in more financially lucrative households, or was the responsibility of the wife.
Today these are little more than antiques, and are only needed in homes where a proper heating and ventilation system has not been installed. Modern technology has replaced the bed warmer in many ways, which is a good thing since placing hot coals anywhere outside a fireplace is simply unsafe.
The term is a bit archaic by some people standards, but even today the phrase is also usable when referring to an individual who is either literally or metaphorically keeping another person's bed warm. Say, a mistress or kept woman for example. There are rumors that in one way or another Katherine Harris has been in bed with the Bush Campaign for the presidency. This is just as well. Many could also claim that President Bill Clinton has been Hillary Clinton's bed warmer for years.