I guess The Slit-Mouthed Woman is a Japanese urban legend or something (you can look it up if you want; I'm not here to do no research). She's some ghost lady with long black hair and a slit-mouth, appropriately enough. I don't know the significance of the slit-mouth (maybe it's just supposed to be creepy), but it reminds me of the autopsy photo of the Black Dahlia. Don't worry, I won't post it. I'll just say that there's something extra unsettling about some maniac slitting the mouth of a victim, maybe because it doesn't seem to serve any purpose. It sorta makes sense for a killer to dismember a corpse so they can litter the limbs around the neighborhood and make it harder to identify or make the body fit in a steamer trunk or something. Just a couple of tips for all you...oh wait, I've said too much. Never mind.
Anyway, there have been several movies revolving around this creepy character, and the dumbest and trashiest is surely The Slit-Mouthed Woman from 2005, directed by Takaaki Hashiguchi. The story involves people having sex in an abandoned hospital. They are interrupted by the slit-mouthed woman, who then kills them. This is repeated several times. Basically, that's the entire movie. Sorry for spoiling it.
The movie works as (very) low exploitation, like a Japanese horror version of some early 90's Skinemax cheesecake movie. However, it does aim for the stars in one particular scene. Namely, the tongue bath bit. Shannon Tweed was no doubt on both the receiving and giving end of some righteous tongue baths during her Skinemax days, but I can say with confidence that this tongue bath is the greatest in cinema history, and possibly even in human history. Maybe I'm still in the Olympic spirit, but this actress deserves a medal folks. Anyway, here's a GIF of it.