Meaning that you have an erection at the present time or talking about one in the past. The 'Shtonk' being the erection an the 'on' meaning you have it, so a Shtonk off would of course mean you dont have one.
A peculiar type of long, wet fart that can only made while engaged in the mutual joys of picking your nose and taking a huge dump. It makes you laugh hard--the ultimatemulti-task.