ETAOIN SHRDLU was used on old typesetting machines (like linotype) to indicate a line with a mistake in it, that should be deleted before printing. It was easier to delete a whole line than fix one or two letters. For example, if I put in "Faster than a spending bullet" I'd add "ETAOIN SHRDLU to the line, and start the next line "Faster than a speeding bullet, more powerful..." ETAOIN SHRDLU were the first two lines on the keyboard, corresponding to the most commonly used letters. (This has been "Welcome to my useless-information-filled mind.")
Link to the book: *** Link No Longer Exists: From Here to Insanity 11 ***