water pillow; water-filled pillow
A pillow filled with water, often used for cooling the head during fever or hot weather.
熱があるときは水枕を使うと気持ちいい。
When you have a fever, using a water pillow feels good.
夏の夜、水枕をして寝ると涼しく感じる。
On summer nights, sleeping with a water pillow feels cool.
Compound of 水 (water) and 枕 (pillow). The exact historical derivation is uncertain, but the word straightforwardly describes a pillow filled with water.