to run into a room and jump on the room's occupants while naked or mostly naked while yelling "PORN NINJA!" and "I'M A KARATE DUDE!" at the top of your lungs. rubbing of your naked bits onto the person in the room is strongly advised.
doing ninja-like flying kicks is also appropriate.
made popular by Fall Out Boy's Pete Wentz in the video Release the Bats.
usually done when drunk, high, or just crazy.
"i was laying on my bed reading a book and Sally came in and porn ninjaed me!"
"Really? to what level?"
"she rubbed her naked body all over me and did flying kicks at my head!"