To be in an inebriated state of drunkeness, but still able to form a sly grin and light a cigarette to impress your friends. Put more simply, another way of saying "I'm wasted".
Can also be used when not drunk at all purely for humour value, or in anticipation of a drinking session.
We thought the OP meant he was trying to remember something, but after he slipped and linked us to the site he was visiting we realized "wracking his brian" meant he was jacking off!