: Methods for creating slang I am interested in creating slang or a vernacular for a particular group in my story. I want it to be distinctive and a definite marker for in-group/out-group, but

I think the answer has to come from who your characters are, and why they are using slang. Essentially slang is an in-group word-game. It's a way of distinguishing insiders from outsiders. It can also be a low-key form of resistance to authority.

So it depends on your group. Techie slang is filled with acronyms and shortenings. It's a way of showing off your familiarity with arcane knowledge. On the other hand, teen slang is designed to annoy adults, has an anti-authority slant, and is often filled with arbitrary words that need to be memorized rather than figured out. Many years ago, in Kenya, I learned a version of the local slang that was filled with a playfully corrupt mix of English and Swahili. It was the anti-traditionalist language of young global citizens, and it was equally an affront to both formal, school-learned English and to the native languages of the villages (and their older inhabitants).

Slang can be crude or elegant, witty or offensive, rule based or arbitrary. It's all about the group that uses it.

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Just look how real life slangs form, there are some common patterns.
To name a few:

Portmanteaus and clipping: fan fiction -> fanfic -> fic, software -> warez.

Abbreviations and acronyms made into words: Product End of Life -> "they EOLed the product, we have to upgrade now".

Complex concepts replaced with references: "if you get caught, they will 94 you" (where 94 is the number of a paragraph in e.g. a legal act).

Also, as a reader, I would like the invented slang to fit the characters who use it. People like spaceship pilots or magicians are unlikely to develop a slang just to look distinct and detect outsiders, since it's skill (or ability) that defines whether one is a member of the group or not, but they are likely to have slang words for common objects and concepts they encounter in their daily life, especially if "correct" names for those concepts are lenghty.
To the contrary, school kids are more likely to invent just a few words for common concepts and some words to hide the meaning of their conversations from adults (cigarette -> cancer etc.).

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Although I can remember as a teenager spending ages learning to say words backwards to fit in with a particular group, for example 'good' becomes 'doog', I am not sure I would use that method in writing. It really annoyed outsiders at the time and I think it would annoy readers.

I would focus on just coming up with one or two words that mean 'good' and 'bad'. For example, when at university, we would often say about something we really liked: 'That's salad.' When I was younger it was: 'That's tops.' The number of variations for just these two categories is huge.

The other category of word you might like to think about is words to describe a member of the opposite sex who is thought to be attractive.

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I would say simply pick a really specific way to change the language, and then change it that way consistently.

Like with your example, it uses words that rhyme with the word they mean in English, in order to essentially create a new language.

Pig Latin takes the first part of a word in English, puts it at the end and adds "-ay" as a suffix.

So I would take a language, which could be English, and modify it in order to create a new one. As long is it's consistent, it should be decipherable.

It's essentially a form of verbal encryption when you think about it. With Pig Latin, you have the the word (for example "amscray"), and then the key to decode it (take the "-ay" off the end, then put the last part as the first part), and you have the word "scram".

So think of a specific "key" in order for the characters to be able to decipher each others language, let's say taking the important words and adding the word "in" in the middle, and swapping the beginning and end.

Then your characters can say "Is the pon-in-wea den-in-hi inside the uck-in-tru?" Which would translate to "Is the weapon hidden inside the truck?"

To expand my answer, it could be a good jumping off point for having an 'in' language. Eventually, common words will just be changed for some reason or other.

For example, using the rule for changing words that I said, eventually someone might create a new weapon and call it a 'ponin', a shortened version of 'pon-in-wea'.

People will only ever create their own language to serve a purpose. Whether that purpose is to befuddle outsiders, to distinguish members of their group or simply to make things easier, such as getting rid of superfluous words in sentences (think of words like 'don't' as a shortened version of 'do not'), the equation is the same.

Purpose + time = new dialect.

People come up with new words all the time for some reason or other, whether or not they catch on is part of the time portion of the equation. Like you say you want people to identify the in-crowd by the language they use, every single generation of teenagers goes through this exact same ritual.

Kids now apparently (in the UK) describe someone old and creepy as 'a bit UKIP', which is taken directly from the current cultural background, and possibly a misunderstanding of adult things.

So whichever rule you use, you need to think about the time element, and whether the words would realistically catch on. As for where they come from, take anything that people are aware of at the time of its creation and use it for your own ends.

As a footnote, another good example I like is 'Devil's Advocate', which originated from the Catholic Church as a person who argued against the side of God, and took a skeptical view of someone when discussions were ongoing about appointing them as a saint. This has now pervaded into popular culture and is a term commonly used by most.

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You could use verlan.
Words of this slang are made up by switching syllables of a word.

It's not be the best method, but it can be a start.

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