The Collaborative International Dictionary of English v.0.48:

Dunder \Dun"der\, n. [Cf. Sp. redundar to overflow.]
   The lees or dregs of cane juice, used in the distillation of
   rum. [West Indies]
   [1913 Webster]

         The use of dunder in the making of rum answers the
         purpose of yeast in the fermentation of flour. --B.
                                                  Edwards.
   [1913 Webster]