The Captain's Booty is an achievement related to the world event Pirates' Day. Simply make it to the top of the auction house and bank of Booty Bay, speak with Dread Captain DeMeza and choose the gossip option "Here's to you, Dread Captain! Make me a pirate for the day." DeMeza will then buff players with Dread Corsair, making them an honorary member of the crew for 12 hours.
Patch 3.2.0 (04-Aug-2009): Added.