Figuratively used to signify the head. His upper story or garrets are unfurnished; i.e. he is an empty or foolish fellow.
Definition taken from The 1811 Dictionary of the Vulgar Tongue, originally by Francis Grose.
Upper Benjamin * Upping BlockNathan Bailey's 1736 Dictionary of canting and thieving slang
John S. Farmer's collection of canting songs and slang rhymes