Remember how, a couple of years ago, everywhere you turned someone was using the word "fleek" or the phrase "on fleek"? And remember how - about the time it started to show up in mainstream culture on TV and in social media - it was already going away?
This is one of the big problems I have with using full-on slang in a book.
Yes, having your characters use words like "fleek" will definitely let your readers know that your book was written by a hip, trendy, and oh so au courant author in 2015. But what if your book didn't go to print until 2016? By the time the book was out, your oh-so-cool terms had already become last year's news.
Honestly, the same can be said - to a lesser degree - about using current bands, current trends (in fashion or food, for instance), or current cars - but for today we're going to try to stay focused on word choices.
There is definitely a time and a place for using jargon and lingo (two words, which, according to my 1954 Funk & Wagnall dictionary used to mean "gibberish") to set a scene or establish your characters.
Doctors need to use appropriate medical terms. They're not going to say that they need a "tube thing with ear pieces to listen to a heart" instead of saying "stethoscope." (Well, we hope, at least.) And the high school football captain might be expected to talk about plays involving end runs and Hail Mary passes, and not foul shots or homers.
Those kinds of job/person-specific language are important to establishing a character's character, as well as letting us know whether or not we should have faith in what is being said.
When you're writing these characters, you need to be able to talk their talk and walk their walk - even if they talk and walk in different universes (or different genders) than your own.
Unfortunately, this can be a slippery slope when you're working with characters that you're nothing like. It can sometimes feel like the easiest way to make your readers believe you is to throw in a lot of lingo and hope that that makes all the difference. (Drop in a "hematoma" here and a "forceps" there and - poof - you've got a doctor. Right?)
One of the most dangerous - in my opinion - groups for this kind of "slang writing" is teenagers, kids, and young adults.
While doctors have, for years, discussed hematomas and forceps, this is because they are terms that have been around for a while. On the other hand, while you may have heard some teenager on TV use the term "on fleek" while you were writing - there's a good chance that you didn't hear it again.
So what can you do? How can you make your main characters fit in 2018 while writing in 2017?
My recommendation: Start by not trying so hard.
Remember the first time you heard your mom call something "gnarly" and you knew she didn't have any idea what it meant? Or the first time your teacher tried to use the word "groovy" and you cringed? They really wanted to fit in. But they tried too hard.
Instead, aim a bit more for the middle-of-the-road. Try for terms that you've heard enough that they've become slightly mainstream. (After all, that means they might stay around longer than just a month or two on social media.) For instance, oddly enough, "cool" seems to keep standing the test of time if you listen to teenaged conversation.
Want your main character to be someone on the outside or a trailblazer? Maybe throw in some retro terms like "nifty" or "swell." Want her to be completely out on her own? Try making up a new term for her to throw around (Lewis Carroll invented all sorts of new words in describing Alice's adventures, J. M. Barrie - according to some sources, if not all - even invented the name Wendy for Peter Pan, and Shakespeare practically made a living making up words).
The main point, here, is to not mire yourself in terms that will hold your book back in six months. Don't use "Yasss...." when "He shook his head, winked, and gave an approving 'Yes'" could work without making readers cringe about how mid-2017 your book is.
Your editor - and your future readers - will thank you.