A Slippery Johnyy is the act of a sexual partner filling their mouth with your urine and then proceeding to give you a blow job. Once you have blown your load, the extra liquid injection causes an explosion of flavor to burst from their mouth.
Person 1: I heard you hooked up with Tiffany, I heard she was a freak!
Person 2: She really is, she is so freaky, she gave me a Slippery Johnny.