Most of us can think of English words or phrases that, while they might seem to mean the opposite, actually mean the same thing. Take the words flammable and inflammable. How many people have regretted lighting cigarettes in the vicinity of signs warning INFLAMMABLE because they thought the word meant nonflammable?
Or how about the words loose and unloose? Or the phrases "I could care less" and "I couldn't care less?"
Sometimes the same words can mean opposite things in different contexts. Jack Lynch lists a number of examples in his book The Lexicographer's Dilemma. Take the word oversight. It can mean either watching carefully, as in the case of an oversight committee, or not carefully enough.
To dust something can mean either adding something, as in the case of cookies and powdered sugar, or removing something, as in the case of furniture and actual dust.
Does a bimonthly magazine come out twice a month or every other month? Well, take your pick.
Lynch notes that we might encourage people to eat hardily by telling them to either eat up, chow down, tuck in or pig out. How is it possible that each of these phrases means the same thing?
Those of us who grew up speaking English have no trouble at all with such illogical constructions, with the possible exception of those who light fires around INFLAMMABLE signs. But pity those who try to learn the language as adults.