Wiktionary
vb. (context slang vulgar English) To rain heavily.
Usage examples of "piss down".
My bowels had moved during the night, and I was able to get only a few inches from the mess, and I had to piss down my leg.
But oh God-the good mood was rapidly dribbling away like piss down a pant-leg.
Did you piss down your own legs when you saw the feast I laid for you?
They'd walk around the parapet and try to piss down on sleepers in the shade but the prisoners kept alert.
He had a specially constructed ringside booth that kept us protected when the Mexicans in the top tier tossed coins and beer cups full of piss down at the ring, and Jimmy Lennon introduced us during the prefight ceremonies.
The guardsman, Rubmos, also watched that flashing star, as he lifted his leathern kilt to piss down the outer face of the wall.