If the meaning of the slang is obvious from context, no explanation is needed. Don't give it. Seriously, the reader doesn't need every phrase explained. Often context is enough:
"I'm sorry. About... about what I said..."
Sam waved it off. "Nothing. Birds in the wind."
If the meaning is unnecessary to following the story, don't give it. Have you ever read the poem "Jabberwocky"? Half the words don't make any sense at all. A couple you can figure out from context (frabjous, galumphing), but most you just don't need to know (slithy toves, borogroves, tulgey wood, and pretty much everything else). And that's okay. You can understand the story fine without them.
If the meaning is not necessary yet, don't give it. Not every term has to be explained right away, even if it's important later. When it becomes important, the reader won't mind you stopping for a paragraph to explain it. In Air Pirates for example, the word 'jacks' is introduced on p. 15 (see excerpt below):
B'Lasser flashed a foot of sharp steel.There's a little context, but the full explanation doesn't come for another 25 pages. Yet nobody's ever complained.
“Oy, oy, oy!” Dean came running out from the back, hands waving. “No blood! You gotta fight, you take it down the road. Else I call the jacks in here.”
When you finally do need to explain it, there are a number of ways you can do it -- more than I list here, certainly:
1) Include a character who doesn't "get" the slang and needs it explained, or who can at least identify with the reader's confusion:
Hagai looked around. "What did you do with my friend?"
"Easy, lad," Sam said. "I just showed him the way out. I didn't pack him either, if that's what you're flailing about. 'Sides, man like him, float in the dark he would."
Hagai didn't know what that meant. "So... you didn't kill him?"
"Nay, Gai, I didn't kill him." Sam's smile mocked him.
2) Use context. You know that first tip, where if the context is there already you don't need to explain it? It works both ways: if you need to explain it, add the context. For example, "grubbing":
Normally, if Sam wanted a snack, he would've just grubbed it off the shelf while no one was looking. But Crike Cappel, who'd been grubbing a lot longer than Sam had, taught him that he had to establish “legitimacy.”
3) When all else fails, tell. You don't want to do this often, but don't be afraid of it either. You gotta do what you gotta do, right?
Fitch came back a few minutes later with the keys. As he opened Sam's cell door, Sam said, "Where's the guard?"
"Sleeping," which meant he was unconscious.
I know I've been talking about slang, but these modes of introduction work for any made-up terms in any genre. (Though obviously speculative fiction will have more of it).
What else? What other methods have you used or seen to introduce foreign terms without being intrusive?
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