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Every year the latest edition of a dictionary boasts about the new words it includes. I expect we'll see sub-prime in the lists for 2009, and Second Life and Facebook and mouse potato and...It's not difficult to find new words to include because there's plenty of choice: every day some three or four new words appear in English. To see if they've achieved a significant presence, all you have to do is search the internet. Sub-prime has about 20 million hits on Google. It'll be in.

What's much trickier is deciding whether a word should stay in the dictionary. Will the latest words achieve a permanent place in the language, or will they not be known in a few years' time? This is often the fate of slang expressions. Who says daddy-o now? Or jeepers-creepers? You can sense the way the language has moved on when you read someone like P.G. Wodehouse.

It isn't only slang. I once did a study of words that were being fêted as new in the 1960s and included in the dictionaries of the time. More than half of them have gone out of everyday use now. Do you recall Rachmanism, Powellism, peaceniks, dancercise, frugs and flower people? All frequent then. Historical memories today.

It's always been like this. In the 16th century, there was a period when people invented thousands of words with Latin and Greek origins - words such as adnichilate (destitute), eximious (excellent) and suppeditate (supply). Critics called them “ink-horn terms” because you needed a lot of ink to write them down. There were even dictionaries to help people to understand what on earth they meant. Only a small number achieved a permanent place in the language.

But dictionaries are notoriously reluctant to leave words out - for the obvious reason that it's very difficult to say when a word actually goes out of use. You can spot a new word easily; but how do you know that an old word has finally died? Did grody (slang for nasty, dirty) die out in the 1970s or is it still being used in the backstreets of Boston? Indeed, you could argue that old words never die, if people keep hearing them. Hundreds hear David Tennant (aka Hamlet) say: “Things rank and gross in nature possess it merely”, in Stratford-upon-Avon each week. Are rank (excessively luxuriant), gross (coarsely abundant) and merely (in its sense of totally) dead? Or just dormant?

On the whole, dictionaries keep words in either until constraints of space force some pruning or a new editorial broom looks at the word list afresh. That's presumably what happened in the present case, where the editors at Collins have decided that abstergent, agrestic and the others are so rare these days that nobody would ever want to look them up.

Just because they're left out of a dictionary of standard English doesn't mean that they've disappeared from the language, of course. Some of the words remain alive and well in regional dialects.

Looking at the Collins list, I know niddering and skirr are still used in parts of Scotland and the North of England, and fubsy (along with fub, meaning stout) is mentioned in several dialect books. Maybe some of the others are too.

It's a daring decision to leave a word out because you can never predict the future with language. A word or phrase can be obsolescent, then suddenly have its fortunes reversed by being used by some celebrity. Remember the slogan “on your bike” (meaning get up and do something useful, adapted from a 1981 remark by Norman Tebbit)? Or a newspaper headline pushes a spoken word into written prominence. Remember Gotcha (The Sun, during the Falklands war)?

Or attitudes change towards a word, so that one generation loves it and the next hates it and the next loves it again. That's what happens to many words that become politically incorrect, such as black and queer. Who knows? Maybe the reporting of the present set of words will attract fresh interest in some of them.

But whatever has happened to words in the past, the future is going to be very different. The internet is about to change everything in lexicography. In an electronic world, dictionaries can be of unlimited size and nothing disappears. Because pages are time-stamped, the internet is already the largest corpus of attested historical language data we have ever known. In that dictionary, words never die.

Even fatidical, attracting a pathetic 9,600 hits on Google today, will live on. If words could talk, they would say they had finally achieved what they always wanted: immortality.

David Crystal is Professor of Linguistics at Bangor University

Dr Johnson included many learned words in his famous dictionary of 1755. Some had negligible use even in his time, and are certainly long gone now, but others retain a curious appeal:

bedswerver one that is false to the bed

curtain-lecture a reproof given by a wife to her husband in bed

fopdoodle a fool; an insignificant wretch

figure-flinger a pretender to astrology and prediction

nappiness the quality of having a nap

perpotation the act of drinking largely

smellfeast a parasite; one who haunts good tables

traveltainted harassed; fatigued with travel

vaticide a murderer of poets

worldling a mortal set upon profit

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