E. Cobham Brewer 1810–1897. Dictionary of Phrase and Fable. 1898.
Dun’derhead.
A blockhead, or, rather, a muddle-headed person. Dunder is the lees or dregs of wine, etc., more correctly, the overflow of fermented liquors (yeast). (Spanish, redundar, to overflow or froth over.)
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“The use of Dunder in the making of rum answers the purpose of yeast in the fermentation of flour.”—Edwards: West Indies.
