A dirty turtle is when you are fucking a girl (or guy) from behind on the floor. Just as you are about to ejaculate, you run their head into the wall, knocking them out unconcious. Once knocked out, ejaculate all over their stomach, and watch them while they lay on their back helplessly, just like a turtle.
My girlfriend woke up on the floor in a state of confusion after i gave her a dirty turtle.
While fucking a girl from behind, quickly pull out and ejaculate in your hand. After ejaculating continue to fuck her from behind. While rockin it, start saying "peter parker, peter parker." In a state of confusion, your parter will look back and say "What?" At this point, release your fluid at her face while yelling, "SPIDERMAN!"
Cindy didn't know what 'hit' her when i showed her my spiderman!