Verb
piss like a racehorse (third-person singular simple present pisses like a racehorse, present participle pissing like a racehorse, simple past and past participle pissed like a racehorse)
(vulgar, idiomatic) To urinate profusely, especially in reference to a profound need to do so.
Pull over as soon as you can. I need to piss like a racehorse!