The state or condition of having something up your ass. Most often used in reference to the location of a stupid person's head.
Origins: The term "defilade" is used in military tactics to refer to a position where natural surroundings protect units, vehicles or equipment from enemy fire.
Sgt. Slaughter: "Private Pile might be the stupidest person in this company. His head is so far in rectal defilade that he can see his own teeth."
A "Picki Defile" is when your raid wipes due to a single person placing the defile on top of a ton of people (usually near the middle of the platform) when facing the Lich King 25 (Normal OR Heroic) encounter.
I spent some time yesterday cleaning, as the inevitableaccumulation of detritus (and filth) had reached a breaking point, and some serious defilthification was necessary.