I'm digressing into phrases here, but did anyone see the Almodovar film 'Tie me up, tie me down'? Two phrases which seem as though they ought to have opposite meanings, but which actually mean the same thing. I can't remember what the original Spanish title was, but I'm sure the English one is totally untranslatable for non-English speakers.

On the subject of words such as 'wicked' and 'terrific', the meanings of these words haven't actually changed, they have just been adopted as slang and subverted, if you like. The definitions of slang words are notoriously fuzzy - look at 'cool' and 'hot' - the definitions of these two diametrically opposite words are so fuzzy that they overlap.