Wiktionary
hot water bottle
n. A flexible, saclike bottle, usually made of rubber, intended to be filled with hot water and placed on some part of the body for thermotherapy, or simple warming.
Wikipedia
Hot water bottle
A hot water bottle (or hottie) is a container filled with hot water and sealed with a stopper, used to provide warmth, typically while in bed, but also for the application of heat to a specific part of the body.
Usage examples of "hot water bottle".
A love of animals has me involved in animal welfare, and certainly explains why the cats hog my bed and the hot water bottle!
Vera quickly appeared with a hot water bottle wrapped in a towel and put it under my feet.