A nonsensical set of letters that would occasionally appear in newspapers, books, etc. (in the days of hard-set type in publishing). Often assumed to be gibberish Latin. A nonsensical phrase.
Linotype machines had these letters in two rows of 6, in their approximate order of frequency in the English language. Supposedly, lino operators would test their machines by running their fingers down the rows, or use the letters to indicate an error in the typesetting, and occasionally would forget to remove the "phrase" from the piece they were typesetting, and was not uncommon to see printed in newspapers of the day. It appeared often enough that it came to the notice of the general populace, and Walt Kelly even named a character in his comic strip "Pogo" Etaoin Shrdlu. It has also appeared- purposefully- in works of fiction and literature.
(as a "practice run") etaoin shrdlu Today the Queen of England....
(as an error demarcation) Today teh QUeen etaolin shrdlu