docking is the act of putting something in your urethra. In layman's term, its the act of shoving a dildo up your penis hole. a ninja docker is some one who puts their penis up another penis so fast that the other person doesnt even feel it or know what just happened.
guy 1: i'm not sure what just happened but wow does my penis feel awesome.
guy 2: dude you just got ninjadocked
or
look at that dude he is such a ninja docker