Actually, to be precise, dinosaurs didn't piss. Like crocodiles, birds, and other reptiles, they leached all the water possible out of their waste and excreted only uric acid (as opposed to urea, the familiar mammalian form). Uric acid is the white stuff that surrounds the black stuff in bird shit. Dinosaurs probably excreted in a similar manner, depositing their minimal uric acid at the same time as their feces. Some lucky dinosaur feces would go on to become coprolites.

Pseudo_Intellectual informs me the term is slang for petroleum. Heh. 'Course, petroleum comes from plant matter, not dinosaurs.