etaoin shrdlu /ˈɛteɪˌɔɪnˈʃrədluː/ is a nonsense phrase that sometimes appeared in print in the days of "hot type" publishing because of a custom of type-casting machine operators. It appeared often enough to become part of newspaper lore.

It is the approximate order of frequency of the 12 most commonly used letters in the English language.

More information on the Wikipedia page [1] and the Fun With Words website [2]: there are other pages.

See also Lorem ipsum and The quick brown fox jumps over the lazy dog