The answer is that misuse becomes the norm. While it is hard for a lover of language to see it happen, it is true that a dictionary definition will depend on how people use a word or phrase, not its derivation.
Over time, words wander and meanings morph and you could end up in embarrassing hot water if you stuck doggedly to the original meaning only.
"Meat" for example used to mean solid food (as opposed to drink) but now it is more specific. This would mean that cavemen would have gone out hunting solids.
Caveman: Look at the solids I speared for us.
Cavewoman: Well, don't bring them inside dripping blood all over the floor I've just cleaned.
Caveman (a bit stuck for words): The proof of the pudding is in the eating.
"Awful" and "awesome" used to be synonymous but by the early 19th century, "awful" began to take on its current negative associations. These days, of course, the first syllable of "awesome" is irrelevant. Awe is no longer required as, apparently, even a pencil case can be awesome.
"Naughty" is another good example. In the 1300s, it used to mean poor or needy (as in having "naught") but time has led it towards "mischievous", "poorly behaved" or "disobedient".
For those in water-problem districts, another word has undergone a rapid change. "Would you like a cup of tea?" has very quickly morphed into, "Would you like a cup of chlorine?"
Even the "pudding" referred to in the opening expression used to mean something different.
This was no spotted dick or treacle roly poly; this was a dubious mixture of minced meat (in the modern sense), suet, oatmeal and seasonings squeezed into some animal stomach lining in the manner currently favoured for sausages and haggis. One writer has called it a "boiled-up farmyard massacre".
Now, as you can probably imagine, hygiene standards were dodgy all those centuries ago so the only way to test whether the pudding was benign or deadly was to try some. And then to smile or die.
Suddenly, you see, the full version of the adage makes sense. "The proof is in the pudding" does not. So, for the language purist, life is tough. You can accept that language changes but it's hard when so much of it simply defies logic.
I wonder where the words and phrases I have discussed today will be in another couple of hundred years. I suppose the proof of the pudding will be in the eating.
Or, perhaps by then, the adage will be "The proof of the spotted dick is in the consumption thereof".
- Wyn Drabble is a teacher of English, a writer, musician and public speaker.