From Longman Dictionary of Contemporary Englishhot-water bottleˌhot-ˈwater ˌbottle noun [countable] 
DHa rubber container full of hot water, used to make a bed warm
DHa rubber container full of hot water, used to make a bed warmExamples from the Corpus
hot-water bottle• My daughter feels as soft as a marshmallow, as warm as a hot-water bottle.• When Penry arrived with the promised drink he also brought a hot-water bottle.• Helen made tea and filled a hot-water bottle for Theresa.• More than once a hot-water bottle filled with cider burst in the back of a closet, for a chambermaid to find.• Alternatively, she could lie on her stomach with a hot wet towel or a hot-water bottle placed on her buttocks.• There was nothing more to do tonight except to fill her hot-water bottle.• The rubber hot-water bottle was a very modern idea at the time the house was built. 