If there is anything we can definitively say about words is that they change meaning, often without warning. Why they do is not under dispute: they change to stock dictionaries with ``obscure'' or ``obsolete'' definitions so obnoxious people can insist that whatever they say actually means the opposite, and you're the fool for not realizing they were using a word in a fashion seen as recently as 1698. You are, but at this point you should know better and the words are just an excuse.
How words change meaning is the interesting question. The most exciting technique is graphitic fission. The most dramatic example of this process was in 1378 when a team lead by Geoffrey Chaucer subjected the word ``deer'' to high-participle bombardment in a confined gerund-lined chamber. In the explosion ``deer'' was separated from its meaning of ``anything that is not a bird or a fish and yet which humans will eat, as long as it isn't a plant'', with only the remaining ``Bambi-like life form'' surviving intact.
More, the back-scattering caused the Great Vowel Shift, in which the ``a'' became the ``e'', the ``i'' became the ``e'', the ``e'' which had been the ``a'' tried changing over to being the ``j'' when it found the ``i'' tromping all over it, and the ``j'' was ejected from the vowels entirely, only to land in a light dusty coating all over the German nations. This technique is not without its perils: a 1754 attempt to force such a fission on the word ``meat'' resulted in war with Spain, although to be fair that sort of thing happened a lot in the old days.
Not as exciting but resulting in fewer isotopes and short-lived phonemes is a protective camouflage, in which a word starts meaning its opposite so that the person using the word will not be slugged by the person the word is being used against. For example, ``nice'', neatly enough, started with the meaning ``if you continue acting like that, someone is going to fardel you with a bare bodkin, and will earn a medal for the effort''.
Just what was meant by this was obscure, since most were agreed that a ``fardel'' is that little plastic container with covers in which sticks of butter are put until that becomes too much work and you leave them in the wax paper wrapping. But it certainly sounded like a threat, so between 1425 and 1994 ``nice'' tried to sound less provocative. As a result people generally approve now of being called nice, which makes the bodkin all that much more surprising.
And sometimes a word will go from pleasant to insulting in a show of mere perverseness.
Another process is reverse mitosis, in which one word discovers secretes a cytoplasm-dissolving compound to envelop and devour another word's definitions. Shorter words are better at this, as their small diameters make them more permeable and provide a greater surface tension, reducing the risk of spontaneous fission. So while it may seem unfair that ``run'' contains the definitions of over 10,374 formerly separate words and shows no signs of exploding anytime soon, it actually is unfair, and there's nothing we can do.
While these processes eliminate some words, new words are formed all the time, as nearly any set of letters not currently in use could meet the franchise fee and become an expansion word. Sometimes an expansion word will be absorbed and then become an expansion word another time. English is already on its fourth ``gossip'', for example, and shows no signs of stopping, however hard we try.
While this word-evolution makes a sentence confusing if its words are in the middle of changing, there is a benefit. The meanings of all the words in any sentence will gradually evolve to every possible meanings. Therefore, you only ever have to write one thing and eventually it will mean whatever anyone might want. This is what makes it so dangerous to be an author, or a reader, or a word. It's not much easier being a sentence, and very few people would be happy as paragraphs for long. So, don't be.
Trivia: A ``nickname'' is a corruption of an ``ekename'', where the ``eke'' means ``an addition or extension'', ``name'' means ``name'', and the ``n'' was slid over from the article ``an'' by mistake. Source: Webster's Dictionary of Word Origins, Editor Frederick C Mish.
Currently Reading: Nature's Building Blocks: An A-Z Guide To The Elements, John Emsley.