I occasionally teach a class about using Microsoft Word. In one of the class exercises, students are asked to format a page, and the instructions tell them to "outdent" a heading. After I got several questions about that each class, I realized that lots of people have no idea what the term means.
Outdent is the opposite of indent. Once you know that, it's easy to see how the in- of indent can become out- when you want to do the opposite of indenting — that is, when you want to move text toward the edge of the document rather than away from it. Some poking around makes it clear that it's an unusual term (it doesn't show up in many dictionaries), and is used only in technical discussions about formatting text.
I was on the other end of this situation when I began taking guitar classes a few years ago. As we worked on different songs, the teacher occasionally referred to the outro, a term I'd never heard, but which other students seemed unfazed by. It was a surprisingly long time before I grasped that the outro was the opposite of the intro — namely, it's the closing portion of the song. Unlike outdent, outro has been around at least 50 years and has entries in many dictionaries.
Outdent and outro are examples of what I think of as "unexpected opposites" — they're opposites for words that you didn't know even have an opposite. Who would have guessed, for example, that we have a word that's the opposite of intro?
I've found some other examples of these unexpected opposites. For example, when I watched the movie Argo, about the audacious plan to rescue some Americans caught up in the Iranian Revolution of 1979, I learned the term exfil, which is short for exfiltrate. Based on context, I pieced together that exfiltrate is the opposite of infiltrate — to exfiltrate someone is to remove them from a dangerous situation, "usu. surreptitiously," as the OED puts it. I have since run across the term in spy novels, although it does seem to date only from about 1980.
Everyone probably knows that someone or something on a blacklist is banned or excluded or otherwise unwelcome. Perhaps you don't know that the opposite of blacklist is whitelist, which is an explicit list of approved items. I see these terms a lot in the context of computer security — for example, if you've ever marked email from someone as "not junk" or "not spam," you've whitelisted that sender.
A pretty recent example of an unexpected opposite is the term to unfriend. This is the antonym of a verb that's itself pretty recent — to friend, as used on Facebook. (Unfriend was New Oxford American Dictionary's Word of the Year for 2009, as reported by Ben Zimmer.)
In my reading in computer literature, I've found other unexpected opposites. Think for a moment about what the opposite might be of to duplicate. One possible opposite is deduplicate, meaning to remove duplicate information. When I used to manage mailing lists for my kids' school, I spent a lot of time deduplicating those lists.
Along those lines, what would you expect the term denormalize to mean? Admittedly, this is a specialized term. In a database, to normalize is to design the database so that it doesn't contain redundant data — for example, in a normalized database, you store a customer's address just once, and you don't re-enter the address each time you enter an order for that customer. It turns out that under some circumstances (typically when a process needs to be fast), it can make sense to undo this design principle and to go ahead and keep redundant copies of the data — that is, to denormalize the data.
Here's another: what's the opposite of stateless? Again, this is something I see in the computer world. A familiar example is a Google search. When you send a search request to Google, Google does its thing, sends you a page of results, and then forgets all about you — that is, it keeps no state about you. The state information about your search, like what term you're searching for and what page you're on, is actually in the stuff at the end of the URL in the browser. So Google searches are stateless. But other processes do store state, like a word processor that remembers what document you edited last. What's a term for the opposite of stateless? Believe it or not, in my world, we use the term stateful.
Another unexpected opposite comes from the realm of free software. Most software is distributed under copyright, which puts restrictions on how it can be used and distributed. But people in the free-software communities wanted the opposite: a license that not only provides free access to software, but makes sure no one can later put restrictions on it. The term that emerged was a kind of opposite of copyright: copyleft.
I'm sure that I know other examples of these unexpected opposites, but they only seem unexpected when you first encounter them, so I've probably gotten used to them. So I'll ask readers: what are some other examples that you've encountered of unexpected opposites?