The Thin Pirate was an associate of the Fat Pirate (actualy big-boned). After the Battle of Plunder Island, their ship was near Plunder Island collecting debris and various items that fell into the sea, intending to sell them to King André. The Thin Pirate saw a pair of nice smoking boots floating, actually the remains of zombie pirate LeChuck, after his accident with the Voodoo Cannonball, which he collected with a hook.
As he threw the boots into the hold with the other loot, he witnessed LeChuck's manifestation as a demon-Pirate.
The two pirates soon became undead, being the first of LeChuck's new Army of the Undead, and set sail for Monkey Island.