A blockhead, or, rather, a muddle-headed person. Dunder, is the less or dregs of wine, etc., more correctly, the overflow of fermented liquors (yeast). (Spanish, re-dundar, to overflow or froth over.)
“The use of Dunder in the making of rum answers the purpose of yeast in the fermentation of flour.” —Edwards: West Indies.
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894