from Fancyclopedia 2 ca. 1959
The first two rows of letters on a linotype machine, used in many connections, e.g., to designate a small printing press owned by Wollheim and Michel, at one time.

from Fancyclopedia 2 Supplement ca. 1960:
A meaning left unmentioned: this was the title of a subzine/SAPSzine which burst without warning onto the Fifth Fandom scene and in a few issues established quite a reputation. Only after the abrupt total disappearance of magazine, editors, publishers and contributors was it learned that the whole thing had been an English class project of the Bronx High School of Science, and the people involved not eager neofen in search of egoboo, but eager students in search of passing grades.

"ETAOIN SHRDLU" were chosen for the top row because they were the twelve most commonly used letters in English). A typewriter's keyboard arrangement is called QWERTYUIOP, chosen to make typing as slow as possible.

Also the title of a 1942 short story by Fredric Brown.