It is when you have been on an all day drinking binge, with all the trimmings. i.e. : 12 pints of strong larger at lunch time, an afternoon curry followed by more larger in the evening, topped off by a kebab with extra chili sauce, then straight home to bed. In the morning when emptying your bowls you will feel like you have shit a kidney ! And probably would have done !
God, i just shit a kidney ! answer, oh, have you just had a shidney ?