Green’s Dictionary of Slang

hulver-headed adj.

stupid.

1785, 1788, 1796		Grose Classical Dict. of the Vulgar Tongue.
c.1786		Grose Classical Dict. of the Vulgar Tongue ms. additions n.p.: Hulver headed signifys a solid Impenetrable head. Hulver signifyiong Holly (in the Norfolk Dialect) a hard & solid wood.
1811		Lex. Balatronicum.