It's perpetually crucial to understand the definition of someone's words and why they ascribe that definition to that word. However, we should never get hung arguing on the former; the latter is what matters.
In all my writings, it's important to understand I don't believe a word has an intrinsic definition, only the meaning ascribed by human speaking that language. In other words, I don't think any word has some "pure," "divine," or "provable" definition given by God, Science, or Reason and that its "true" definition will remain regardless of what people think and any erring from the true definition is false no matter its usage.
I disagree. A word is just air blowing through one person's mouth that sounds similar enough to the air thru someone else's mouth which later were interpreted into squiggles of ink on a page. There are two kinds of words: common and jargon. Common words are entirely defined based on what the vast majority of people think it means. Jargon words are words created for a specific context to avoid the changeability, incoherence, and the vocabulary limitations of common words; a jargon word receives a definition when a group of people decide that in their special context, word X will mean Y regardless of what X means in the outside world. Even the definition of jargon words are arbitrary and subject to change.
Let's consider several definitions, both common and jargon. In 2008 subprime mortgages became critical to the United States economy. However, "subprime," "sub-prime," and "sub prime" were not listed in either the OED or Merriam-Webster dictionaries. A spokesperson and lexicographer for Merriam-Webster stated that absense from the dictionary doesn't mean subprime or any word is not a real word. "Mouse" has changed recently to refer primarily to a compute mouse and refering to a rodent less often as computers are more common. The jargon definition of planet changed in 2006 to disclude Pluto. A committee was formed that oversaw the change and considered various proposals, one proposal which would include more than the original 9 bodies. The final proposal was included because it was the most helpful in classifying bodies. There was no serious thought to nastolgic arguments to keep Pluto a planet just because it was before or that God made planets and humans can't change the meaning.
The Greek word from which we derive X means Y, therefore X also means Y. Definitions change. They are not defined by past definitions.
X was made up 50 years ago, so Y is false. Every word was made up at some point, whether a year ago or a thousand years ago. Even the word "true," at some point was only 50 years old. Time tests how useful X is and how long a language remain in usage, but it does not test the validity of Y.
There has never been any word, X or otherwise, that means Y, therefore Y is false. This incorrectly assumes that a concept can exist without a word and that any concept that is true has already been known and articulated into a word.
X means Y and not Z, therefore it cannot change to include Z. This implies words are incapable of changing, which history shows is false indeed. If Z is more useful than Y alone, then X will change regardless of our opinion.
X means Y and not Z, therefore it should not change to include Z. This implies either that X has an intrinsic meaning or that a word should not change. Both are false. If Z is a useful concept, the right thing is to find it a word. If Y and Z are fundamentally different, then Z should find a word other than X, but if Y and Z are similar and an additional word would be difficult to create, we shold combine it. Of course, people don't sit down and decide definitions other than jargon. The "should we" question isn't useful unless it's possible to choose which is rarely the case.