noun
pocket heater; hand warmer; heater pack
Refers to a small, portable heat source, typically a disposable chemical pack or reusable warmer, used to warm hands or body in cold weather.
寒い日はカイロをポケットに入れています。
On cold days, I keep a hand warmer in my pocket.
使い捨てカイロはコンビニで買えます。
Disposable heater packs can be bought at convenience stores.
A hot-water bottle used for warming a bed or body, typically larger and reusable, unlike a disposable pocket heater.
From 懐 (futokoro, 'pocket' or 'bosom') and 炉 (ro, 'hearth' or 'furnace'), literally a 'pocket hearth'. The word is now almost always written in kana.