Words that mean “VERY GOOD” but also mean (or used to mean) “Not to be Believed”:
Fantastic: I.e., fantasy, i.e., not true.
Incredible: I.e., not credible, i.e., not believable.
Unbelievable: I’ll let you work this one out.
Fabulous: From a fable, i.e., not true.
Other types of “Very Good” words:
Amazing: Shares an ancestor with “maze.” Both meant something that confuses.
Awesome: Inspiring awe.
Wonderful: Inspiring wonder. (Incidentally, “wonderful” is my answer to people who say that “Hopefully” simply must refer to something done “while full of hope” because of logic. Logic has nothing to do with language, and long may it last. The movie was wonderful, but I wasn’t wonderful while watching the movie.)
Terrific: Shares an ancestor with “terrify.” Both meant frightening.
Of course, there are lots of other words that I won’t bother with today. A smattering from the thesaurus:
sensational, outstanding, superb, super, excellent, first-rate, first-class, dazzling, breathtaking; great, ace, wicked, brilliant, killer.