Interested in Language
It is rather amusing to know that authoritarian and authoritative have somehow different (if not opposing) meaning.
I looked in the "Online Etymology Dictionary" and found this:
1879 (adj.), "favoring imposed order over freedom," from authority. Cf. authoritative, which originally had this meaning to itself. Noun in the sense of one advocating or practicing such governance is from 1883.
c.1600, "dictatorial" (a sense now restricted to authoritarian), from authority (q.v.). Meaning "possessing authority" is recorded from 1650s; that of "proceeding from proper authority" is from 1809.
Well, it seems that both words had the same meaning, but then authoritative in the 1950 started to mean the opposite!
Nowadays, authoritative is used to mean "trust worthy" as when we say "Oxford is an authoritative dictionary".
So what happened exactly to make this word change drastically!
It doesn't mean the opposite to me- it means that it possesses authority, so we trust the OED because it commands respect legitimately, rather than through hectoring, etc. To me it's a useful distinction, and having one word with those two meanings would lead inevitably to ambiguity, so a new noun was the solution.