Wiktionary
piss like a racehorse
vb. (context vulgar idiomatic English) To urinate profusely.
Usage examples of "piss like a racehorse".
The fourth thing was that he had to, you should pardon the expression, piss like a racehorse.
And at last, I think a more basic truth must have overwhelmed the rest of his calculations: the tape was off his mouth, but he was still wearing the straitjacket and probably he had to piss like a racehorse.
He had to piss like a racehorse but for almost a minute nothing happened.