You can quickly draw a hidden weapon to make a deadly strike.
Prerequisites: Sleight of Hand 8 ranks, Quick Draw.
Benefit: If an opponent provokes an attack of opportunity from you, you can draw a weapon that you have successfully concealed using Sleight of Hand as an immediate action to deliver the attack of opportunity with that weapon. That opponent is treated as flat-footed against the attack with the concealed weapon.
Source: Complete Scoundrel, page 89.