This is a topic I have been interested in for a while.
I've long understood that English works by 'conventional usage'. That is to say that a word means whatever everybody understands it to mean, as opposed to what some controlling body wants it to mean.
Hence, a word can change meaning. Something which goes 'against' English grammar can become acceptable, such as a split infinitive or using 'and' at the beginning of a sentence.
If everybody started using the word 'dinosaur' for breakfast (see: http://en.wikipedia.org/wiki/Wordplay_(The_Twilight_Zone)), then dinosaur would become an acceptable word to use for breakfast.
The Oxford Essential Guide to Writing has this to say (page 243):
Words Are Not Endowed with Fixed and "Proper" Meanings
When people object to how someone else uses a word, they often say, "That isn't its proper meaning." The word disinterested, for example, is frequently employed in the sense of "uninterested," and those who dislike this usage argue that the proper meaning of disinterested is "objective, unbiased. "
In such arguments "proper meaning" generally signifies a meaning sanctioned by past usage or even by the original, etymological sense of the word. But the dogma that words come to us out of the past with proper meanings—fixed and immutable—is a fallacy. The only meanings a word has are those that the speakers of the language choose to give it. If enough speakers of English use disinterested to mean "uninterested," then by definition they have given that meaning to the word.
Those who take a conservative attitude toward language have the right, even the duty, to resist changes which they feel lessen the efficiency of English. They should, however, base their resistance upon demonstrating why the change does make for inefficiency, not upon an authoritarian claim that it violates proper meaning.
As a user of words you should be guided by consensus, that is, the meanings agreed upon by your fellow speakers of English, the meanings recorded in dictionaries. We shall look at what dictionaries do in Chapter 29. For now, simply understand that dictionary definitions are not "proper meanings" but succinct statements of consensual meanings.
In most cases the consensus emerges from an activity in which individual language users participate without knowing that they are, in effect, defining words. The person who says "I was disinterested in the lecture" does not intend to alter the meaning of disinterested. He or she has simply heard the word used this way before.
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