As seen in the Korean web novel 'FFF-Class Trashero', a
sexual partner (usually in the case of the main character
Kang Han Soo referring to a woman), basically a 'bed warmer'.
"A beauty, whose body was ideal to be the highest-grade
hot water bottle, thus shouted while looking down from the top of a majestic
fortress covered by ice and snow." - FFF-Class Trashero Chapter 65
"His girlfriend, who had played the role of a hot water bottle, had disappeared, while he was kidnapped to the continents of
Fantasia again." - FFF-Class Trashero Chapter 75