This is the 4842nd most frequent Japanese word.
湯たんぽ
"湯たんぽ" is a hot water bottle used for warming.
湯たんぽ is used here as a noun referring to a hot water bottle used for warming up bedding for nighttime comfort.
夜は湯たんぽを布団に入れて暖かくします。
I put a hot water bottle in the futon at night to keep warm.
湯たんぽ is again used as a noun indicating the object that is being filled with hot water.
湯たんぽにお湯を入れるときは注意が必要です。
You need to be careful when filling the hot water bottle with water.
湯たんぽ serves as a noun describing the essential object used during past winter nights for warmth.