hot-water bottle

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun A stoppered container, usually made of plastic or rubber, that is filled with hot water and applied to a part of the body for warmth.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• 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