A stupid person; one without brains. The allusion is to a wig maker's dummy or tête à perruque, on which he fits his wigs.
“Your wit will not so soon out as another man's will, `tis strongly wedged up in a block-head” —Shakespeare Coriolanus, ii. 3.
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894