Turtle Knock
When two penis heads touch together.
This most commonly happens during a threesome via an act called "double penetration" when two males accidentally pull out at the same time and knock the tips of their penises together. "Hey bro, I didn't mean to turtle knock you last night, It was a total accident."
turtle knocking
The act of fucking someone and mistakenly giving them a hickey
Hey Amanda, by the look of Wes' neck you guys did some Turtle Knocking last night.