A Rusty Rocket happens when you do a girl or guy in the ass with a dildo so hard, it jars the shit loose and it sticks to the dildo. You then take the dildo out and throw it as hard as you can at her face while making a bomb noise with your mouth.
Jonny and Jenny were fooling around and Jonny says, "Do you know what a Rusty Rocket is? Jenny says no. Jonny starts to perform the act of ass-fucking Jenny with the dildo. She got so excited by this act that she let loose and shit on the dildo. Jonny then says, "You shit on the dildo you slut!" He then takes it and chucks it at her face and says as hard as he can and exclaimed with pride, "Boom!!"