The top of the bell curve of one's drunkeness. One usually mentions to a friend, often off-to-the-side or discreetly that they are "half in the wrapper". This usually occurs long after it's apparent to casual observers.
The point at which the downhill slide to passing out is inevitable.
Wrapper refers to a blanket or comforter one seeks in the last moments when there's nothing left to do but curl up and pass out.
Check it out! Brian's trying to act all cool with those chicks but he's half in the wrapper.