What? You read that right.
ETAOIN SHRDLU are the 12 most common letters in the English language. They are also the two “home” columns on a Linotype keyboard.
The phrase was fairly commonly seen in type, accidentally of course, up until the 1960s and it was a mistake. A typist would run his hand down the columns to indicate there was a mistake in the line and that it should be eliminated, but proofreaders make mistakes and things go to print with misplaced semi-colons or (at times hilariously) missing commas, and so ETAOIN SHRDLU would sometimes make it into my grandparents’ newspapers.*
The term ETAOIN SHRDLU has come to mean something that is nonsensical or absurd. There is a play I stage managed in college called The Adding Machine, about a man who is replaced by an adding machine after 25 years on the job and then kills his boss, that has a character named Etaoin Shrdlu.
For more on the frequency of letters, in itself a really interesting breakdown of English, see here.