In-jokes. The secret language of people who’ve known each other or been part of the same subset of a subculture for a while, they serve a writer or roleplayer as a way of illustrating a strong shared history between one or more characters. But for people going a little more meta, they’re also an excellent way of illustrating how slang in general evolves. After all, really strong in-jokes don’t require them to explain what they’re saying to each other, nor even to use a full sentence—an offhand reference to exploding obelisks can carry just as much meaning. So how does in-joke slang—and slang in general—evolve?
An in-joke begins with a situation, circumstance or quality—basically the thing that gives it its meaning. It might be a general class of things, like someone behaving stupidly, or it might be a very specific incident, but whatever it is, it’s something memorable enough, and probably capable enough of being specifically analogized, to be worth using as an in-joke.
Then there’s the summarization: someone commenting on the situation in such a way as to encapsulate it, or an existing sentence like a song line or movie quote, that happens to fit. For instance, there’s a bit of in-joke slang in my household that originally sprang up from my boyfriend’s and my difficulties in dealing with small children (he and they don’t get along well, I do well with them but not for prolonged periods of time). He summarized it all with a joke about how when we settle down, we should raise 2.5 conures, itself a reference to the old “2.5 children” statistic. (For those of you who’ve never met a conure, they’re small birds.)
That brings us to the trigger: the situation to which the in-joke analogizes, such that invoking the in-joke can explain the situation. In our case, it’s small children acting up—tantrums at the supermarket or the video game store, someone messing up the puzzle at the library yet again, all the myriad ways. The long-form in our case is “This is why we should raise 2.5 conures instead.”
From there, the summarization gets shortened down, all the unnecessary detail pruned off to leave only the parts necessary to invoke the joke. For a characteristic behavior, it might be the name of the person or the group likeliest to do that particular behavior, either “pulling a [name]” or just verbing the name directly. Other things might shorten other ways, though usually there’ll be a noun, maybe a verb, and possibly an adjective involved. In our case, the final form of the line is “2.5 conures”, since that calls up the full image on its own; when the trigger comes up, one of us will eyeroll and mutter “2.5 conures”, usually sounding like either a curse or a nondirectional prayer. It doesn’t make a jot of sense to other people, but it makes perfect sense to us.
So if you want to get across the strong history between two characters, think about how their in-jokes are likely to have evolved. Not only will that make it that more convincing, but it might save you a bit of exposition (they aren’t explaining the joke every time they use it, just when someone asks) and possibly add a little humor.