A Turtle Cake is when a man has a small penis that is also uncircumcised that resembles a frightened turtle and is put between the ass cheeks of an unsuspecting victim, thus the turtle is between the cakes.

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.