There is a fairly widespread folk etymology for this word, involving the Emperor Napoleon. Supposedly, at some point during his Prussian wars the Bonaparte became peckish, but the only food that could be scared up was a loaf of black bread; at this, the Corsican, used to croissants and pains riches, said »Bah c'est pain pour Niquel!« and promptly fed the bread to his warhorse, Niquel or Nickel. The words »pain pour Niquel« were then transformed by German pronunciation to »pumpernickel«.

However, any child can see at once that this is nonsense since, as everyone knows, Napoleon's warhorse was named Marengo.