An old man chopping wood “misses” his target and splits open the back of his wife’s skull. Oops. The strike is horizontal for some reason. He must like a challenge. She sure must kneel in strange places.
Also, it’s his second wife, who starved the man’s daughter to death after giving all the food to her own kids.
The wound doesn’t heal, but grows into a ravenous demonic mouth. The woman takes on the hunger of the girl she killed.
Origin stories vary, from shapeshifting hags and spiders to curses brought on via crazy shit as in the above.
Found a great Futakuchi-onna Pinterest collection.
In The Changing Things, a paranoid woman named Darlene believes she was Futakuchi-onna in a former life. She seeks out surgery that will modify her body to conform to her self-image. At first the mouth is non-functional and built with marmoset incisors. Later she seeks out experimental reconstruction for providing a pathway for food from her new mouth to her esophagus. Not quite sure what to do about the handsy hairs.