Believe it or not, 'etaoin shrdlu' is perfectly good English, if its inclusion in the Oxford English Dictionary is anything to go by. It can be pronounced 'eh-ta-o-in shird-loo'. It might look like an acronym, but it isn't one. Neither is it an abbreviation or a contraction. Perhaps it's a phrase from some foreign language that has found its way into English, like 'shibboleth', or 'ukase', or 'Imam Bayildi'?

No, it is simply one version of what is believed to be the order of the most frequently used letters in English. The full sequence is

etaoin shrdlu cmfgyp wbvkxj qz

'e' being the most frequent and 'z' the least. This sort of ranking of relative frequency of letters likely to be encountered is very important for the science of cryptography, of course - but that is not how the phrase got into the language.

It comes from a remarkable invention which first came out in 1886 and revolutionised the world of typesetting - the Linotype machine. This is a large and complicated device that sets type for printing, and does it one line at a time. The Linotype operator uses a keyboard to key in the letters, then each line set up has hot metal - molten lead at about 550°F (about 288°C) - run into it to form a 'slug', which then goes into a 'galley' for printing.

Unlike the keyboard used for typewriters - the familiar 'qwerty' layout - the Linotype keyboard used the 'etaoin shrdlu' layout. The first row of keys (reading downwards) was 'etaoin', the next row 'shrdlu', and so on. Now one of the problems with Linotype was that there was no backspace key, and correcting errors was a rather complicated affair. When something went wrong, it was often simpler and quicker for the Linotype operator to fill the rest of the line with any old rubbish. He would then key the whole line afresh, leaving it to the proofreader to get rid of the 'rubbish' line.

The easy way to fill up a line was to run the fingers down a row of keys. So the fill-in could easily be something like

shrdlushrdlushrdlushrdlushrdlushrdlu.

This would be easy enough for the proofreader to spot, yet on a number of occasions they would miss it, and the garbled line would be printed. This is how the phrase 'etaoin shrdlu' came into the language. There are collections of clippings of such errors in print.

The phrase was picked up and became something of a cult, as these things do. For example, the great American writer and humourist James Thurber mentions 'Etaoin' in a story. And there is a science fiction story by Fredric Brown called Etaoin Shrdlu, in which an artificially intelligent Linotype machine is able to understand all the text it sets. Of course, the Linotype tries to take over the world. The machine is finally defeated by the hero of the story, who has the brilliant idea of getting the machine to typeset every possible Buddhist text. As the machine thus comes to learn more and more about Buddhism, it naturally comes to see the essential futility of trying to take over anything, and ends up calmly meditating on what in a Linotype machine passes for its navel.

SHRDLU is a program for understanding natural language, written by Terry Winograd at the MIT Artificial Intelligence Laboratory in 1968 - 1970. It is described in Winograd's dissertation with the title Procedures as a Representation for Data in a Computer Program for Understanding Natural Language. It was published as a full issue of the journal Cognitive Psychology Vol. 3 No 1, 1972, and as a book, Understanding Natural Language (Academic Press, 1972).

The complete phrase 'etaoin shrdlu', or each half of it, has come to refer to text that is nonsense or absurd. The British scientist, cellist and polymath Denys Parsons compiled The Best of Shrdlu from his four previous collections of hilarious misprints and oddball stories that have appeared in print. Parsons described the character Gobfrey Shrdlu as the 'malicious spirit with an irrepressible sense of humour who lurks at the elbow of tired journalists and printers with disastrous consequences' - a sort of latter-day Titivillus.