Pisswater prince is the name that Tyrion gives the tanner’s son from Pisswater Bend who was swapped with Prince Aegon Targaryen when he was an infant. His mother died birthing him and his father sold him to Lord Varys for a jug of Arbor gold - he had other sons but had never tasted Arbor gold. Varys gave the pisswater prince to Elia and carried Aegon away for his own safety. 
During the Sack of King's Landing, Ser Gregor Clegane scaled Maegor's Holdfast and entered the nursery of the infant Prince Aegon, son of Prince Rhaegar, and killed the baby present by dashing the boy's head against a wall. He then raped Princess Elia Martell, his hands still covered with the blood of the murdered baby.