1750-60; < GermanPumpernickel orig., an opprobrious name for anyone considered disagreeable, equivalent to pumper(n) to break wind + Nickel hypocoristic from of Nikolaus Nicholas (cf. nickel); presumably applied to the bread from its effect on the digestive system
"dark rye bread," 1756, from Ger. (Westphalian dialect) Pumpernickel (1663), originally an abusive nickname for a stupid person, from pumpern "to break wind" + Nickel "goblin, lout, rascal," from proper name Niklaus. An earlier Ger. name for it was krankbrot, lit. "sick-bread."