Sister Mary Katherine walks into Jack's liquor store and says, "Oh Jack, give me a pint o' the brandy."
"Sister Mary Katherine," exclaims Jack, "I could never do that! I've never sold alcohol to a nun in my life!"
"Oh Jack," she responds, "it's only for the Mother Superior." Her voice drops. "It helps her constipation, you know."
So, Jack sells her the brandy. Later that night as Jack walks home, he passes the convent. Sister Mary Katherine is singing and dancing, whirling around and flapping her arms like a bird. A crowd gathers, so Jack pushes through and exclaims, "Sister Mary Katherine! For shame! You told me the brandy was for the Mother Superior's constipation!"
Sister Mary Katherine replies, "And so it is, me lad, so it is. When she sees me, she's going to sh*t!"