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.
Did you see ben? he had a huge shtonk on over dave, what a gay!
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 ultimate multi-task.
I heard Steve's loud shtork through the bathroom walls and he laughed so hard that he fell off the bowl!