pee like a racehorse

Etymology:

• From horse racing. Racehorses are thought to race faster when their bladders are emptiest, so racehorses are injected with diuretics (Lasix) right before they race. This draws fluids into their bladder and forces them to urinate right before they race. The thinking is that a horse is at its lightest weight when its bladder is emptiest. So ideally if the horse urinates right before racing it will run faster and be more likely to win the race. The expression "pee like a race horse" then became used as more colorful way to describe how urgently a person needs to urinate by calling this graphic to mind.