adv. Twice a month
adv. Every two months
Although not an auto-antonym in the purest sense of the word, bimonthly deserves mention. Simply put, this is one of the most worthless words in the English language (yes I know there are no degrees of worthlessness). It serves one purpose, to inform the reader the frequency with which something happens, but it fails to do that in every single occasion. Looking this up in the O.E.D., I could not even tell what the original meaning of the word was, because all of the 19th century sentences made reference to a “bi-monthly meeting” or a “bi-monthly publication.” The only way to make the word clear is to make it redundant.
Also useless is the sister word, biweekly, which suffers from the same failure. When I first subscribed to ESPN the Magazine, a mistake in itself, I initially thought it was an outrageously cheap price for a subscription that came out twice a week. It took only a few seconds to clarify the situation, but I was dumbstruck by the nature of the word.
At the scope of a year, the system is much more logical. Biannual means twice a year; it is synonymic with semiannual. Biennial means occuring every two years. Two different times, two different words. The way it should be.
This has me thinking: is there any other word in the English language that is still in common usage that has lost all sense of informativeness? Peruse would fall into this category, but at least you can sometimes get a sense of the meaning from context.