CEDICT
hot water bottle; hot water bag (made of rubber)
Wiktionary
橡皮製的袋子。内装热水，可作为保暖、热敷之用。
重編國語辭典
装热水来保暖的橡胶袋子
維基詞典
A hot water bottle is something you must buy to prepare for the winter months.
Source: Tatoeba