The act of standing while urinating between a persons legs while the said person is sitting on the lou while the other said person is urinating in the lou simultaneously.
Johnny and Sarah are waiting in line for the lou and they both decide to save time by using the restroom together. Sarah sits on the lou and proceeds to urinate while Johnny begins to urinate in between Sarah's legs. Ergo Johnny and Sarah save precious party time between pee breaks. Hence the term waterloo.