Except if you say something is a ghoul, a new player will know A) this is a dangerous man-shaped thing and B) it almost certainly wants to eat me.
Are you sure about that? Because I honestly am not so sure these (formerly?) broadly known facts about D&D-ized terms that are also real-world terms -- are so broadly known. If they ever were.
Particularly once we get away from a term like ghoul which, almost regardless of culture/myth/story, wanted to consume you in some fashion.
If you say to a new player, "You see a kobold" -- what does that new player know about it? Quite possibly nothing! Quite possibly "kobold" is the trigger for the "yugoloth" / "gesundheit" joke.
"Goblin"? OK, what's a goblin? It's
wildly different in folklore, and it's
wildly different in editions of D&D, and its
wildly different in videogames.
"Ghost"? This has to have thousands of different manifestations (
seewhatIddidthere) across real-world human myth. Yet the D&D ghost is
super specific and needs to be defined to make sense to new players -- it's probably not exactly like what they imagine.
I agree with your original point that we need to define our terms. The fact that all definitions of words inevitably consist of other words, each of which may also need to be defined, shouldn't deter us from making the attempt at all. But we should be quite purposeful in how we do it.