What is the term for a word that has come to mean the opposite of its original meaning"
Hmmm.... good question! Not the same as a contronym, a word that has a homograph which is an antonym. Dust can mean dust, or to remove dust, fast can mean fixed in place or moving quickly, and so on.
I'll bet there is a word to answer the question, though.
As for an example, "awful" is awfully close to having come to mean the opposite of what it originally meant. There are others which are close- nice originally meant "foolish"- but I can't remember one which has come to mean its complete opposite. I'm sure there are some, probably in specific fields (music for example).