【 カイロ・kairo 】pocket warmer, hand warmer
The kanji for カイロ is 懐炉.
The kunyomi (Japanese reading) of 懐 is ふところ, which means "chest," and 炉 is the kanji for "fireplace, oven, furnace."
Back in the old Edo days, they used to wrap a heated stone in a piece of cloth—this was called 温石 (おんじゃく)—and slipped them in the chest area under their kimono to keep warm (since there were no pockets, of course). 温石 was the beginning of the 懐炉, a portable warmer to warm the chest area, which then evolved into the pocket warmer.