Noun
a stoppered receptacle (usually made of rubber) that is to be filled with hot water and used for warming a bed or parts of the body
Source: WordNethot-water bottle
Once I showed up at my sister's with a baby rabbit I had bought from some children because its ears were cold. I put the rabbit on a hot water bottle and massaged its ears for quite a while. After all, I knew that all healthy animals had warm ears. Juliette Gordon Low