According to Japanese beliefs, himamushi nyūdō is a grotesque yōkai which lives under floorboards and crawls out at night time. It vaguely resembles a Buddhist monk, but it has a long neck, sharp claws, a body covered in thick, dark hair, and a very long tongue which it uses to lap up the oil from lamps.

Himamushi nyūdō bothers people who are working hard or studying late at night by jumping out of the darkness and scaring them. Although it doesn’t directly attack people, its presence is disturbing enough. It blows out the lights suddenly, and it licks up the precious lamp oil, making it difficult to continue working.

According to Toriyama Sekien’s description of this yōkai in Konjaku hyakki shūi, himamushi nyūdō is born from those who were lazy in life, carelessly wasting time from birth to death.

The word “oven bug” in its name is probably a reference to cockroaches. The hima kanji in this yōkai’s name can also be read kama — and likely refers to the kamado, a traditional Japanese oven. Cockroaches have quite a few nicknames in Japanese; among them himushi (“fire bug”) and hitorimushi (“lamp bug”), both of which sound similar to himamushi. Cockroaches and other pests would have fed on the fish oil in Edo period oil lamps; just like this yōkai. Cockroaches live in dark, warm spaces, such as underneath a kamado; just like this yōkai. And they crawl out of the floorboards to scare those working late at night; just like this yōkai.

Himamushi nyūdō’s name contains a number of puns. According to Toriyama Sekien, it was originally called himamushiyo nyūdō (“monks who waste time at night”). Over the years, the pronunciation gradually morphed, and it became associated with hemamusho nyūdō — a popular Edo period word doodle in which a monk is drawn using the characters in its name: ヘマムショ入道. The connection with this cartoon character would probably have amused readers during Sekien’s time.