A dirtyturtle 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 dirtyturtle.
The occasional ring of shit stuck under the head of your dick after you sodomize someone. aka ring around the collar.
Eric-"I finally got to Nicole's back door last night."
Ted-"So how was it?"
Eric-"Ok I guess,but I pulled out a dirty turtleneck."
Ted- "Goddamn ring around the collar."