The letters on type-casting machine keyboards (such as Linotype and Intertype) were arranged by descending letter frequency to speed up the mechanical operation of the machine, so lower-case e-t-a-o-i-n and s-h-r-d-l-u were the first two columns on the left side of the keyboard.
Each key would cause a brass 'matrix' (an individual letter mold) from the corresponding slot in a font magazine to drop and be added to a line mold. After a line had been cast, the constituent matrices of its mold were returned to the font magazine.
If a mistake was made, the line could theoretically be corrected by hand in the assembler area. However, manipulating the matrices by hand within the partially assembled line was time-consuming and presented the chance of disturbing important adjustments. It was much quicker to fill out the bad line and discard the resulting line of text, than it was to redo it properly.
To make the line long enough to proceed through the machine, operators would finish it by running a finger down the first columns of the keyboard, which created a pattern that could be easily noticed by proofreaders. Occasionally such a line would be overlooked and make its way into print.
Appearances outside typography
The phrase has gained enough notability to appear outside typography, including:
Variations of "Etaoin Shrdlu" are used in the title of some works, including "Etaoin Shrdlu", a 1942 short story by Fredric Brown about a sentient Linotype machine (a sequel, "Son of Etaoin Shrdlu", was written by others in 1981); the 1945 whimsical short story "Etaoin and Shrdlu" by Anthony Armstrong which ends "And Sir Etaoin and Shrdlu married and lived so happily ever after that whenever you come across Etaoin's name even today it's generally followed by Shrdlu's".; a 50-year history of the National Press Club (USA) published in 1958 titled Shrdlu – An Affectionate Chronicle; and The Best of Shrdlu, a collection by Denys Parsons of humorous misprints and double meanings from newspapers that Parsons ascribed to a mischievous character named Gobfrey Shrdlu, referring to collectors of them as Shrdlologists.
The rogue-like video game Nethack uses randomized names for unidentified magic scrolls; one of these names is ETAOIN SHRDLU.
"Molten Fairies: Sprites of a Newspaper" appeared in Perth's "The Daily News" in 1922.
Shrdlu (Norman Shrdlu) is listed as the composer of "Jam Blues", cut 1 on the 1951 Norman Granz-produced jazz album released in 1990 as Charlie Parker Jam Session. This appears to be a joke on Granz's part as Norman Shrdlu is credited in several Parker (and other) tunes that are jam sessions rather than compositions.