Many years ago I heard a linguistics professor use a term to describe when negative words evolve to express positive opinions. For example, African Americans began in the 1970s to say "bad" when approving of someone or something.
The only other examples of adjectives I can think of come from modern slang, as in "sick", "ill" and "mad". But there's a long history of well-known adverbs like "awfully", "terribly" and "horribly" that are used for positive emphasis, as in "It smells awfully delicious."
The professor didn't use "antagonym", "Janus word", etc., which describe words that also mean their opposite; he was discussing the phenomenon in which words take on their opposite meaning, like "literally" did when it recently came to mean "figuratively" (alas!).
Are there any linguists here who might know something about that term or concept?