Means to have to defecate or have a bowel movement or to have to take a dump. Mainly used for an emergency situation, where the movement in imminent, but can be used for any announcement of your condition.
Remember that jalepeno burrito I had earlier, with the ice cream sundae and Heinekins for dessert? Well, Mr Hankey is knocking on the door about now!!
I've been sick all day, and I think Mr Hankey is knocking on the door again.