What is the meaning of Dunder?

The lees or dregs of cane juice, used in the distillation of rum.

Distillery effluent.

A double underscore, __.

thunder

blast it, goddamn (used to show displeasure or disappointment)

a deep, loud noise like from a lightning bolt; booming, thunder

great, popping; potent (of a drug)

thunder

Alternative form of dhunder

Source: wiktionary.org