Getting masturbated manually while driving a motor vehicle
Laura: Didn't you see that car just pull out in front of you
Trevor: No, I wasn't paying attention, you were carjacking me at the time
The act of sneakily masturbating onto a car door in such a way that the owner of said car will stick his hand in the mess before realizing it is there when he attempts entry into his vehicle.
The carjacker will typically attempt to do this in a time and place where he can watch the results of his escapades when the owner arrives, without the owner being aware he is being observed.
Hey man, I went out carjacking today and got enough footage to fill my memory stick!