“I gotta piss like a racehorse”
When and where did it originate? Why a racehorse?
For some reason, after the Herculean exertion of a full-out dash around a dirt track, a horse needs to urinate. It’s convenient, too, because officials need a sample, for drug testing, from the winning horse. It’s not too convenient for the swells standing around the winner, as they’ll need a shoeshine.
I may be wrong about this, but I always took it to be an statement to the effect that the rate and force of flow will be rather greater than usual due to the urgent nature of the task.
Take it from one who’s had to jump back FAST – a horse, especially a male horse, pisses a Niagara, even without having run a race. As AskNott says, exertion can cause the production to leave an even greater impression on bystanders.
Some racehorses, though, don’t produce a sample right away, and have to be held in a special place after the race, if they’ve won, until they spew it forth. This can get very tedious, so some trainers teach horses to piss on command. How? Not difficult – just say a particular word, or whistle a particular set of notes, whenever you observe the horse begin pissing, and the Pavlovian effect will do the rest.