Definition of "dunder"

dunder¹

/ˈdʌndə(ɹ)/

noun
uncountable
1. (Caribbean) The lees or dregs of cane juice, used in the distillation of rum.
  Quotations
  The use of dunder in the making of rum, answers the purpose of yeast in the fermentation of flour.
  1793, Bryan Edwards, The History, Civil and Commercial, of the British Colonies in the West Indies‎, Dublin: Luke White, Volume II, Book V, Chapter 2, p. 231
2. (Australia) Distillery effluent.

dunder²

/ˈdʌndə(ɹ)/

noun
plural dunders
1. (programming, informal) A double underscore, __.
  Quotations
  Python has a dunder method, __iter__, that defines what the behavior is for looping over an instance.
  2012, Matt Harrison, Treading on Python, volume 1, page 101