Now I wonder if there's a general term for words that ought to be antonyms but are actually synonyms. An example would be flammable and inflammable.

I used to work on a San Francisco team with a Zurich counterpart, mostly staffed with people of Italian descent. There was a quite confusing situation once involving a misunderstanding of my use of "terrific"; they thought I was calling something horrifyingly bad. Another such situation went something like this:

Me: "I tried what you suggested and now everything's hunky-dory."

Them, 12 hours later: "Well if you're going to complain about my suggestion, I wish you'd have at least explained what was wrong with it.

A passage in the "Vogue book of manners" from the '30's insisted that one never, ever say that a woman's dress was terrific, with literally no explanation.

Terrific is my favorite autoantonym, but at this point the “terror causing” definition is little more than entomology trivia.

To quote the old joke, confusing entomology and etymology bugs me in ways I cannot put into words.

That's terrific! I LOLed.

Similar to 厉害 in Chinese - it often translates as terrible, but mostly used to mean great.

"Awesome" and "Aweful" have similar etymologoies: both mean, "Inspiring or creating awe". But one has morphed into, "really good", and the other into "really bad".

Likewise "terrible" and "terrific".

I believe "terrible" in the sense given for 厉害 is as in "I am Oz, the great and terrible", not the ordinary sense of the word.

This. It can be used more colloquially than the equivalent meaning of "terrible," where it's closer to something like "awesome." It can also generally used as a marker of intensity. In no case does it mean "bad at something."

Can you give an example where it means terrible? I can't think of any.

That's just terrorific!