A Chinese term used often in online novels.
It refers to someone who is secretly evil, sly or manipulative. Secretively is the key point: most people won't realize the person's true nature or the person will portray a different image.
The term exists because someone will only know what that person's true nature is only after "being eaten up."

Briefly, "the realization that each random passerby is living a life as vivid and complex as your own." Originally from the Dictionary of Obscure Sorrows, which has a lot more beautiful neologism definitions like this you might enjoy.