From Monkey Island wiki
The Pappapisshu Bush is a plant found on Plunder Island with sharp thorns. A note from the Plunder Island Naturalist Society reads: "Discovered by Plunder Island's indigenous peoples, this bush is named after a native word meaning 'Youch!'" Guybrush can use one of the thorns in a peashooter when he is trapped in quicksand. If he reads the note before taking the thorn, he will cry "Pappapisshu!" when he touches the plant.
In addition Guybrush will say pappapisshu any time afterwards instead of Ouch. The Cabaña Boy will also yell "Pappapisshu!" when Guybrush snaps him with a towel. Guybrush will also utter the line in Tales of Monkey Island Chapter 5: "Rise of the Pirate God" when he tries reaching for the Cursed Cutlass of Kaflu as a reanimated corpse on LeChuck's ship.