ETAOIN SHRDLU is the approximate order of frequency of the twelve most commonly used letters in the English language.
As the letters on Linotype keyboards were arranged by letter frequency, ETAOIN SHRDLU were the first two vertical columns on the left side of the keyboard.
Piecework or Publishing Linotype operators who had made a mistake or typing error whilst setting a line of type would have to fix it by hand in the matrix assembly or the quickest way for some operators to fix the problem was to run a finger down the keys, thus creating the nonsense phrase ETAOIN SHRDLU.
A good operator would then remove the incorrect slug after it was cast.
Lines that weren’t removed by the operator would normally have been caught by the proofreaders at the galley proof stage.
If the slug with the error made it as far as the compositors, the distinctive set of letters served to quickly identify it for removal.
Occasionally, however, the phrase would be overlooked and get printed erroneously.
This happened often enough for the nonsense phrase ETAOIN SHRDLU to be listed in the Oxford English Dictionary and in the Random House Webster’s Unabridged Dictionary.
Running your fingers down the keys was also a good way to check that the matrix and spaceband keys were functioning smoothly.
ETAOIN SHRDLU also became part of the folklore and mystery of hot metal newspapers.