I am trying to determine the origin of the old saying "pizz like a race horse".......anyone know?

**** like a racehorse

Meaning: An urgent need to urinate.

Example: Stop the car at the next tree, I have to **** like a racehorse.

Origin: Horses, it turns out, don't always feel comfortable urinating just anywhere. Show horses and racehorses spend a great deal of time in their pens and come to feel safe and secure there. They don't like to urinate outside of those pens and in many cases won't.

In fact show and racehorses are frequently returned to their pens to allow them to urinate. Hence racehorses are often walking around outside of their pens with an urgent need to urinate.

Thanks to Neuticals

Alternatively, this phrase originated in the practice (currently illegal, I believe) of giving diuretics to racehorses. The horse would then urinate substantially and drop a few pounds in the process.

Voila! A lighter, faster, somewhat dehydrated horse.

Steve Lawler & Joy Riggs

It usually means you take a very long pee.

a race horse can pee a long time...

Good luck

Sorry I cant help you