When a compositor (one who sets type in a composing "stick" let the neatly set type fall, it becomes "pi." I have always thought of it as in making a pie by mixing up ingredients--apples, raisins, cinnamon. The jumbled type is then thrown into a "hell box," and I think the origin of that is pretty clear. After the typesetting is done, an apprentice may be set to sorting pi back into the proper type drawers. In the days before the Linotype and other typesetting machines, it required a lot of typesetters, broadly called "printers" although they may never get closer to the press than delivering type to the compositor's stone table, to prepare a daily newspaper. So in the 1850s-70s it was a job for a large number of high school age boys, much as being a gas jockey was in the 1940s, or a fast-food clerk (which included high school age girls) in more recent years.