The head. His garret, or upper story, is empty, or unfurnished; i.e. he has no brains, he is a fool.
Definition taken from The 1811 Dictionary of the Vulgar Tongue, originally by Francis Grose.
garnish * garret electionNathan Bailey's 1721 Dictionary of canting and thieving slang
John S. Farmer's collection of canting songs and slang rhymes