these have come up here several times (AHD4 gives sanction and cleave as examples). A couple of word guys have proposed neologisms to stand for the concept: contranym (Richard Lederer) and antagonym (Charles Ellis).
It's not spelled out anywhere I can see, but I take it by a Janus word you mean a "two-faced" one that is capable of being used not just equivocally but antonymically, i.e. with exactly opposite meanings? Debatable whether
sanction is precisely in this category. Near enough I suppose, but the positive usage means official approval for something and the negative usage means the penalty or trade consequences of the official disapproval of something, rather than the disapproval itself. The negative usage is almost invariably in the plural, also, as in "trade sanctions against Iran."
Does it have to be a noun-noun or verb-verb correlation? Or can it be a verb-noun antonymic usage?
