At first glance, I thought this was yet another Engrish mistake. But after a closer look, I realized that "Devil's Fart" is correctly translated from the characters above it, which meant... Google time!
Interestingly enough here's what I discovered:
"Pumpernickel" originates from German words that loosely mean...
You guessed it! "Devil's Fart." Observe.
The German word "Pumpen" was used to describe flatulence, and the coarse texture of the bread encouraged such behavior from those who consumed it.
The German word "Nickel" is short for "Nicholas," a name often given to goblins, or the devil, or anything dark in nature. Had to do some additional googling to confirm this one -- especially with all the St. Nicholas stories out there.
Therefore when you put it all together, combined with the bread's common flatulence side-effect, you have Pumpernickel Bread -- aka Devil's Fart.