I've often warned people against the etymological fallacy -- the erroneous assertion "that what a word "really means" [or should mean -EV] is whatever it once meant long ago, perhaps even in another language."

But I hadn't realized, until a few days ago, that the word "etymology" itself comes from a Greek root meaning "true," and hence signifies "studying the true meanings and values of words." So the etymological fallacy is what happens when one fallaciously equates the etymology of a word with what the etymology of "etymology" would suggest -- the true meaning of a word.