A printer's devil was an apprentice in a printing establishment who performed a number of tasks, such as mixing tubs of ink and fetching type. A number of famous men served as printer's devils in their youth, including Ambrose Bierce, Benjamin Franklin, Thomas Jefferson, Walt Whitman, Mark Twain, Warren Harding, John Kellogg, Lyndon Johnson and Joseph Lyons.
Printer's devil has been ascribed to the fact that printer's apprentices would inevitably have parts of their skin stained black from contact with the ink involved in the printing process. As black was associated with the "black arts," the apprentice came to be called a devil.
Another origin is linked to the fanciful belief among printers that a special devil haunted every print-shop, performing mischief such as inverting type, misspelling words or removing entire lines of completed type. The apprentice became a substitute source of blame and came to be called a printer's devil by association.
A third source involves a business partner of Johann Gutenberg, John Fust, who sold several of Gutenberg's Bibles to King Louis XI of France and his court officials, representing the bibles as hand-copied manuscripts. When it was discovered that individual letters were identical in appearance, Fust was accused of witchcraft—the red ink text was said to have been written in blood, and Fust was imprisoned. Though Fust was later freed after the bibles' origins were revealed, many still believed he was in league with Satan, thus the phrase.
Another possible origin is ascribed to Aldus Manutius, a well known Venetian printer of the renaissance, and founder of the Aldine Press, who was denounced by detractors for practicing the black arts (early printing was long associated with devilry). The assistant to Manutius was a young boy of African descent who was accused of being the embodiment of Satan and dubbed the printer's devil.
One likely source stems from the fact that worn-out and broken lead type is thrown into a hellbox, which the printer's devil has the task of sorting out and wheeling into the furnace for smelting down and recasting.
Finally, English tradition links the origin of printer's devil to the assistant of the first English printer and book publisher, William Caxton. Caxton's assistant was named "Deville" which naturally evolved to "devil" over time, as that name was used to describe other printers' apprentices.