"Swag" describes a person who is cool or enviable, or someone can "have swag." But it hasn't been trendy since 2012, teens say.
Now, teens are more likely to express something that's cool as "lit." It's slightly different than swag in that it's not typically used to describe a person; a situation or thing is more likely to be described as "lit."
"Bae" comes from African-American Vernacular English (AAVE) pronunciation of "babe." It used to be the ultra-popular way to refer to your significant other, but teens say it's now overused.
There isn't a trendy word to use instead of "bae," actually. But if you want to call a friend something other than their names, teens recommend "bruh."
Drake popularized "YOLO" in his 2012 song "The Motto." It means "You only live once," and became trendy as a phrase to justify seizing the day.
DJ Khalid takes the credit for 2018's rap-popularized slang: "key" or "major key." This can be used to describe anything that's essential or important.
"Rad" is yet another synonym for cool, and teens say it's totally out-of-touch. In fact, it hasn't been trendy since the 1990s.
A better way to say something is cool is "chill." A person, situation, or thing can be chill, and it doesn't necessarily mean relaxed or calm.
Teens also picked "gnarly" as a word that no one should ever use. It's another synonym of "cool" that dominated in the 70s and 80s.
Use "dope" instead.
"Savage" refers to someone who acts ridiculously and without restraint, but it's usually at least somewhat admiring. They often do what everyone else wants to do if they weren't scared.
But savage is out. If you want to express sympathy with a friend who is in an unfortunate situation or got hurt (perhaps by someone who is savage), teens recommend "oof."
"On fleek" describes something that's perfect. It also came from AAVE, and was popularized by a 2014 Vine video where the user brags about her "eyebrows on fleek."
A trendier word to describe something that's perfect is "GOAT." That's "greatest of all time."