Brown George or rye-bread used by Westphalian peasants.
His Transparency of Pumpernickel.
So the Times
satirised the minor German princes, “whose
ninety men and ten drummers constituted their whole embattled host on
the parade-ground before their palace; and whose revenue was supplied
by a percentage on the tax levied on strangers at the Pumpernickel
Kursaal.” (July 18, 1866.)
Thackeray was author of the phrase.
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894