When one is finished masurbating or having sex, and they don't "wash-up" the head of their penis will form a protective coat of dried semen over time. Effectively creating a "Turtle-Head".
Turtle-Head is when you are holding in a turd for so long you can feel the pooppoking out of your butt like how a turtle pokes its head of of its shell.
"oh no" john said to himself during the meeting, "I can feel the turtle-head!"
another way of saying "skid marks". "Turtle head poking out" often refereed to as Needing to take a dump so bad that a lil pokes out. "Graffiti" represents the skid marks in one underwear via "turtle head".