I’ve never understood terror and terrific. Terrific means something good. How did it ever come to mean something good? I’m serious, I want someone to tell me. 
Same way it was sick and awesome.
I.e. intense
Supposedly the original meaning of the word ambulance was applied to two people carrying a stretcher. So sort of related.
I’d argue that this one doesn’t count - as “I could care less” is really a misstatement. that has simply become mainstream.
“head over heels” e.g. “she fell head over heels in love” has always struck me as a rather stupid expression as well (similar to “could care less”) - as most people’s heads ARE over their heels unless they are acrobats. That doesn’t quite fit into the topic though.
Valuable and Invaluable aren’t really synonyms: a valuable item is worth a lot of money, and you could quote a figure for how much it’s worth. Invaluable, however, means it’s impossible to put a price on the thing. Chances are it’s something that WOULD be worth a lot, if you could only figure that out.
I have no basis for this, but I think the origin of “I could care less” probably came from truncating the first half of the following sentence due to apathy, “I don’t think I could care less!”
Looking at the dictionary definitions, I can imagine how, over time, “exciting or fit to excite fear or awe (i.e. terrifying)” morphed into “extraordinary” morphed into “unusually fine.” It’s kind of the opposite direction from the word “awful” (= “awe-ful”).