Shu and I were talking about words that mean the opposite at times, such as terrific or terribly. You can have a "terrific" headache, or the play was "terrific." You can be terribly sorry about something, or you can write terribly. Shu mentioned that the word "awful" used to mean full of awe, but now has changed to something bad.
We probably have talked about this before, as we've been going for 20 years now. However, do you know of others?
There's the rather dated teen slang of "wicked". "He's a wicked man." and "That's a wicked car."
Almost any positive word reverses its meaning if said in a sarcastic tone.
"So, what do you think of the plan?" "Great!"
And, only tangentially related I have always liked this quote from Terry Pratchett's Lords And Ladies"
"Elves are wonderful. They provoke wonder.
Elves are marvelous. They cause marvels.
Elves are fantastic. They create fantasies.
Elves are glamourous. They project glamour.
Elves are enchanting. They weave enchantment.
Elves are terrific. They Beget terror.
The thing about words is that meanings twist like a snake, and if you want to find snakes look for them behind words that have changed their meaning.
No One ever said that elves are nice.
Elves are bad."
"No man but a blockhead ever wrote except for money." Samuel Johnson.
Recalling Isaac Asimov on the subject, refuting the theory that a negative is a negative, and a positive is a positive, and a double negative is a positive, but there's no way a double positive can be a negative. Until a voice came from the back of the room, saying "Yeah, yeah!"
Which I suppose exemplifies your comment on sarcasm making anything into its opposite --
How about "neo-liberal?"