Our own Justin E. H. Smith in his eponymous blog:
Antonyms, of course, are pairs of words that have meanings opposite to each other. Autoantonyms, in turn, are single words that themselves can mean either one thing or its opposite. This can happen either by convergence –e.g., the English verb 'to cleave' comes from two separate but similar Anglo-Saxon verbs, and today can mean either 'to separate' or 'to latch on'– or it can happen through a cleavage, so to speak, within a single lexical item– thus 'to dust' means either to remove the dust from something or to cover something, perhaps that very thing, with dust or a dust-like substance. You might think that autoantonyms of the latter sort are rare birds in the dictionary, but in fact they are all over the place, particularly when the opposition between motion and rest is in question. Thus the adjective 'fast' means both 'swift with respect to motion' and 'bolted down', i.e., 'motionless'. A little reflection will also convince you that most prepositions are capable of autoantonymy. This in fact may have happened to you already: when confronted by a well-intentioned fund-raiser in the street, who tells you that she is raising money 'for breast cancer', does a little part of you not wish to reply: 'Sorry, no, I'm against breast cancer'?
More here.
