Man, was I right.
The last couple of days I've been working on the beginnings of a glossary for my post-apocalypse world. The bad news is it's just as hard as I remember it being the first time around. The good news is, I've figured out some steps to help you (and future-me) start your own fantasy glossary.
1. Determine what feel you want. Old West slang (like Firefly) has a very different feel from pirate slang (like Air Pirates) or Mexican slang or British rhyming slang. Each will flavor your book and your world differently.
2. Research that type of slang. Write down words you like, that sound cool, that are so obscure you think you could use them without most people knowing the source. Even for words you don't like, write it down if it means something you think you'll need. In that case, pay attention to where the term came from (if possible) and see if you can use the same method to create something new.
For example, when researching pirate slang, I wrote down "grog" and "booty." The terms were too well-known for me to use them (and, in fact, their origins made it impossible for me to use them realistically), but they helped me get the right feel for my own.
3. Know your world's origins and metaphors. If your world is at all based on Earth, you'll want to think about how language might have evolved. For example, Firefly mixed Chinese phrases and swear words with English based on the idea that the two "mega-cultures" had combined.
And Earth or not, every world has its own metaphors. What is (or used to be) important in your world? An icy world might have snow and cold metaphors (like, maybe they'd say "Toasty!" instead of "Cool!"). An agricultural society might use farming or animal terms -- like "groundhog" for someone who's never flown before -- while a city-planet might not know what a groundhog is (though maybe they used to know, and it's become a dead metaphor!).
4. Make up some basic terms. Once you've collected everything above, start with some or all of the following (apologies in advance for some of the examples):
- A greeting ("Hey!" "What's up?" "Are you well?")
- A couple of honorifics (Mister, Miss, Your Honor, Madame, Sensei)
- A term between friends (buddy, bro, mate)
- One or two insults (bastard, prick, rat orphan)
- One or two oaths (oh my God, damn it, sh-t)
- A positive epithet (cool, awesome, rad, pure guava)
Once you've got the world in your head and a start to the language, it gets easier. You build momentum for thinking up future phrases, and the bigger your glossary (because you are writing them all down, aren't you?), the more momentum you have and the easier it gets.
At least I hope it gets easier. Otherwise I got a loooooong road ahead of me.