You know the passage:
There's glory for you!'
'I don't know what you mean by "glory",' Alice said.
Humpty Dumpty smiled contemptuously. 'Of course you don't — till I tell you. I meant "there's a nice knock-down argument for you!"'
'But "glory" doesn't mean "a nice knock-down argument",' Alice objected.
'When I use a word,' Humpty Dumpty said, in rather a scornful tone, 'it means just what I choose it to mean — neither more nor less.'
'The question is,' said Alice, 'whether you can make words mean so many different things.'
'The question is,' said Humpty Dumpty, 'which is to be master — that's all.'
Always good for a chuckle at Humpty Dumpty's linguistic solipsism. But the Rev. Mr. Dodgson was a clever writer who embedded considerable intellectual sophistication in what are nominally stories for children.
At a deeper level here, Humpty Dumpty recognizes the plasticity of language. He is absolutely right: Words mean what we (collectively) choose them to mean, and we are the masters. That is why one word, set, can bear scores of meanings. That is why words like sanction and cleave can carry opposite meanings. That is why a word like nice can have had in a long career many different meanings: "slutty," "sweet," "precise."
This plasticity is what Samuel Johnson was mistaken about in his "Plan of an English Dictionary" in 1747. Oh, he recognized that "speech was not formed by an analogy sent from heaven. It did not descend to us in a state of uniformity and perfection, but was produced by necessity, and enlarged by accident, and is, therefore, composed of dissimilar parts, thrown together by negligence, by affectation, by learning or by ignorance." But his stated intention was to produce "a dictionary by which the pronunciation of our language may be fixed, and its attainment facilitated; by which its purity may be preserved, its use ascertained, and its duration lengthened."
Purity. Eighteenth-century grammarians' infatuation with the prestige language, Latin, which, being dead, was fixed, led them to think that English, too, could be Latinized and fixed, brought into society with its rude Teutonic origins refined. English could be Eliza Doolittled.
Thus Johnson selected his illustrative quotations from the purest writers of English. His dictionary was to be a model of proper use, retarding the inevitable changes to which living languages are given.
By the time of publication in 1755, Johnson's preface displays a rueful awareness: "Those who have been persuaded to think well of my design, require that it should fix our language, and put a stop to those alterations which time and chance have hitherto been suffered to make in it without opposition. With this consequence I will confess that I flattered myself for a while; but now begin to fear that I have indulged expectation which neither reason nor experience can justify. When we see men grow old and die at a certain time one after another, from century to century, we laugh at the elixir that promises to prolong life to a thousand years; and with equal justice may the lexicographer be derided, who being able to produce no example of a nation that has preserved their words and phrases from mutability, shall imagine that his dictionary can embalm his language, and secure it from corruption and decay, that it is in his power to change sublunary nature, or clear the world at once from folly, vanity, and affectation."
Even so, he achieved a good deal of his original ambition, because dictionaries, combined with broader public education, have had a braking effect on language, at least as it is taught in schools and practiced on formal occasions. And the success of Johnson's dictionary, little short of idolatry, which inspired Noah Webster on these shores, has led to an enduring public perception that dictionaries are not recorders but preceptors, determining what is "official" and "correct." We see this in people who carry on about the Oxford English Dictionary without evidently having made much use of it, talking and writing as if it were the production of an Academie anglaise rather than what it is, an enormous Fibber McGee's closet of the language.
At Slate.com you can find "Are You a Grammar Bully?" in which the pathology of correcting other people's English usage in public is explored. The people who carry on rudely about violations of "rules," some of which are imaginary, and minor slips of spelling or pronunciation are people who like to talk about "purity." I have seen people who fatuously proclaim that, thanks to the Internet and those damned texting Young People, we will all soon be reduced to communicating in simian screeches. Palpable tommyrot, tosh, twaddle. You should not be giving such people deference.
The question remains, which is to be master. It's your language. You should allow yourself to be instructed and advised, but never bullied.