I keep mentally arguing with that woman Douglas2/Unknown referred to in his comment under Transcending One's Era, he had known who believed that any use of the word "man" to mean "human," not only in our era, but in previous generations, signified a person who was intentionally excluding women from his formulation as unimportant. He noted what an effective insulator this was against listening to what anyone writing before last Tuesday had to say.
When properly applied, teaching that dogma is as effective as burning the library of Alexandria -- the best knowledge of those who preceded us is effectively lost, even if the books and words are still right there in front of us.
Hers is an extreme example of what many others fall into that is not only bad linguistics, but in the end, bad thinking. I had a woman tell me once that she wanted to get Noah Webster's 1828 Dictionary for her homeschooled children so that they would be able to look up what a word "really" means. For all of us, what a word "really" means is how we use it, and this will be close to two concepts: what our current group of communications understands as the meaning, intersecting with what we were taught in school and early adulthood is proper usage. That latter includes its own difficulties. I started school over 60 years ago, was taught by older teachers, in New England, with heavy influence of older well-read women who were conscious of their immigrant background and the importance of proper speech to rise in American intellectual society. That idea of "proper" English thus carries a lot of baggage that is 150 years old, not to mention regional bias.
A favorite example I have used before: If you were to go to some distant tribe to learn their language and make a dictionary of it, taking a full year and trying to be thorough, you would eventually be directed to a remote group, possibly older, who spoke what some would call a "pure" form of the language and others would call an "old-fashioned" version. Some of the mainstream people might even have grown up among them and regard them fondly. So you would innocently go to speak to these people, armed with your near-complete notes, ready for publication. But a couple of old guys and old wives would tell you that your dictionary is wrong, and they would tell you why "Anca doesn't really mean cart, but the wheels on the cart, because it comes from the word anga, meaning 'axle.' That new slang term of calling a cart anca comes from the Teoni people downstream who say it that way. That's not a proper was to say 'cart' in our language." You would nod politely, reflect on this interesting aspect of language change, and ignore their input entirely. What the word means is what the four thousand people in the villages understand it to mean, not what some relics who are kicking off soon think.
So too with us. Language changes underneath us at all times. I am personally in favor of retaining older usages as long as possible, to keep them current so that current children can understand what previous generations meant with as little difficulty as possible. I am reading The Fellowship of the Ring to my 13 y/o granddaughter and occasionally have to explain a word, though I minimise this in order to maintain the narrative flow.
A word in a document "really" means what it meant to the community that received it, or even what the author thought it meant to that community. That is usually an unnecessary refinement of the definition, but occasionally matters. What the word has morphed into since that time is interesting, and may shed some light on the meaning, but is largely irrelevant. This works in reverse as well. What a word now means is what the community of hearers understands it to mean. What it used to mean, or what we think it "should" mean, might be misleading. Most frequently, a meaning will be added to a word, sometimes humorously, sometimes with angry political meaning, sometime invisibly, which older speakers will declare "doesn't mean that." Racism has acquired new meanings. It is wise to recognise that when attempting to understand others. However, that does not mean the older meaning has been destroyed. That collection of sounds used to have one meaning, now it has two, and you are well within your rights to use the term as you see fit. Just know your audience. You may have to say "I am using fantastic in the older sense because it captures my shadings of meaning more exactly." And it is fair to say that "the newer usages of marginalised and racist aren't that helpful." The newer version doesn't get to be what the word "really" means either.
Update: My understanding is that Antonin Scalia was and Clarence Thomas is very sticky about this. What the meaning of a word or phrase at the time it was written into the Constitution (or an Amendment) is key. The meanings the words took on later, even meanings that lawyers subsequently settled on in the 19th C, are distractions. I have read arguments that some strict constructionists actually aren't, following language myths and preferred understandings of today, but I don't think that holds against those two justices.