It's a humorous 2D-animated game about Pirates, with the main character named Guybrush Threepwood.
"Papapishu!" is beeing said when Guybrush gets trapped in some quicksand, and needs to get a thorn from a nearby plant to continue and get out the quicksand.
When he gets the thorn he gets pricked by it and he says "Ah! Papapishu!".
However: He only says "Papapishu!" when you read a sign next to the thorn-plant, wich says the following:
-Papapishu Bush: This bush derives it's name from a native word meaning "Ouch".-
From that point on in the game, every time Guybrush is supposed to say "Ouch!", he says "Papapishu!" instead.
Guybrush: "Ah! Papapishu!"