/pum"peuhr nik'euhl/ , n.
a coarse, dark, slightly sour bread made of unbolted rye.
[ 1750-60; Pumpernickel orig., an opprobrious name for anyone considered disagreeable, equiv. 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 ]