A sexual act that involves pulling out of your partner at the last second and quickly fisting them once. Then immediately grabbing the covers and diving under them much like a turtle shell. The final act is to slowly poke your head out of the blankets as if you were a turtle.
B: "I heard you and Jon got pretty rough last night?"
A: "Yeah , I don't know how to really describe it but it kind of felt like a turtlefoot?"