`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.'
Lewis Carroll's Humpty Dumpty is contemptuous and scornful; I hold no brief for his manners.
I do think, though, that there's more to Humpty Dumpty's thesis than is generally granted. After all, how would you finish this sentence: "When I use a word, it means..."
Now, when he uses the word "glory" to mean "a nice knock-down argument," Humpty Dumpty fails to communicate his meaning to Alice. He doesn't mind, since her confusion affords him the chance to be contemptuous, but most of us, most of the time, use words to signify something to another person. We can only succeed in doing this, of course, when the other person knows what we mean by the words we use.
But we don't have to both mean the same thing by the same word. When Jcecil3 uses the word "sexism," he means something like "the systemic oppression of women." When I use it, I mean something like "an act based on inappropriate reference to sex." They are cleary distinct concepts, but as long as we understand each other's meaning, we should be able to avoid "That's sexist!"/"No, it's not!" back-and-forth bickering.
Where I think Humpty Dumpty overplays his position is in the words "neither more nor less." The meaning of many spoken and written words, especially when they don't refer to concrete things, is not precise enough to be "just" one thing, "neither more nor less." This may be one reason why the second step in a discussion after defining terms is distinguishing ideas. If I distinguish "adult" and "child," then whatever definition I might have given for "adult" becomes, not "neither more nor less" than that definition, but "not a child." A particular definition may be vague, but distinctions give clear limits to the vagueness.
What some people try to do, though, is insist, "When you use a word, it means to you just what I choose it to mean." That way lies madness, or at least irritability on the part of the person having meanings forced upon him. Surely, if a word signifies different things to different people, misunderstandings will occur, but a dogged insistence that by your use of a word you can only intend to signify what the word signifies to me is foolish.