Actually...even more interesting than anyone else has said. Racehorses used to be given a dose of Lasix (furosemide), which is a diuretic, before a race so that they would not have bleeding from the nose and lungs during the race (a weird thing that can happen to horses when they really exert themselves, like during a race) by lowering their blood pressure (among other things). Because lasix is also a diuretic, it also caused them to pee A LOT. That is where the phrase originated.