A disposable or reusable hand warmer; smaller and used for pockets or hands, unlike a 湯たんぽ which is larger and meant for bedding or body warmth.
Etymology
From 湯 (hot water) and たんぽ, which is thought to derive from the Portuguese 'tampão' (plug, stopper) or from the Dutch 'tang' (tongs) + 'pot', but the exact origin is uncertain. The kanji 湯婆 are ateji applied later.