Luca Brennan’s friends spent days debating what phrase they’d write on their shirts for their senior panorama photo.

But then Brennan, 17, seized on the two words the teens had used over and over since the end of the summer to push back against adults who annoyed them and friends who they felt acted close-minded.

“OK boomer.”

The final picture, which will appear in the yearbook of Centreville High School in Virginia, showed the group of nine students with the words written in blue and white tape across their chests.

In recent months, the phrase “OK boomer” has become a common retort in the parts of the internet inhabited by teenage and young adult users. On Instagram, the phrase appears as a hashtag alongside memes and artwork mocking the older generation. On Twitter, the phrase is hurled at someone for making an outdated statement. And on TikTok, where it is arguably the most prolific, it appears in artwork, audios and makeup tutorials as a way to mock an older generation, and the hashtag has been viewed on the platform 18 million times.

“I think a big part of why it has caught on is just, like, baby boomers and older people in general love to complain about younger people on the whole,” said Sam Harman, 17, who took part in the “OK boomer” picture. “They’ll call anyone younger than them ‘millennials,’ and doing the same thing to older people by calling them ‘boomers’ is kind of a push back to that.”

The phrase is a culmination of annoyance and frustration at a generation young people perceive to be worsening issues like climate change, political polarization and economic hardship. The 10 teens and young adults who spoke to NBC News about the phrase said “OK boomer” marked a boiling point for Gen Z and younger millennials, who feel pushed around or condescended to by older generations.

The phrase is even being used to sell sweatshirts.

“I feel like it caught on so well because it’s catchy and humorous, but it also got such a big reaction out of the older generation, which gave it its power essentially and caused people to use it more,” Cassidy Carter, 19, said.

“OK boomer” has begun appearing en mass as comments on videos on TikTok that young people feel are made in bad faith or project an outdated point of view. It’s unclear exactly when the phrase was first used and began to gain traction, but many teens say they first began to pick up on it over the summer.

Some teens point to a Soundcloud audio that has also been uploaded to TikTok, which was created by artists Peter Kuli and Jedwill. The Soundcloud audio was posted this month, but a version of the song was posted to Twitter by Jedwill as early as July. Other teens who spoke to NBC News said they began using the phrase after hearing Twitch streamer “CallMeCarsonLIVE” say it. Others said they picked it up from one another.

But regardless of where the phrase originated, its ubiquity among young people is undeniable.

One video on TikTok that exemplifies the rise of “OK boomer” was posted by an adult with the username “TheSnowflakeMelter2024.” In the video, the adult used a filter called “Drag Queen Makeup,” giving him giant red lips and luscious pink hair, to mock teen environmental activist Greta Thunberg and her speech before the United Nations Climate Action Summit last month.

Of more than 200 comments on the video, nearly every one reads “OK boomer.”

TikTok screengrab of comments showing young people using the phrase "OK boomer." TheSnowflakeMelter2024 / via TikTok

“They underestimate us a lot just because of our age and how we’re growing up,” Saptarshi Biswas, 17, said. “They think we’re given everything, but I think another thing they don’t realize is that they're making decisions for our future and they aren’t really taking responsibility for it, and I think ‘OK boomer’ is kind of an accountability check.”

But young people are split as to whether the term is akin to adults calling them “snowflake.” Many say they feel “boomer” is an inoffensive way to brush off criticism from the older generation.

“I don’t think ‘OK boomer’ is a retort on the ‘snowflake’ name-calling,” said Hannah Hill, 20. “It isn’t intended in the malicious way that ‘snowflake’ is aimed at younger generations. It’s a funny way the younger people can laugh off the entitlement of some baby boomers. It is a humorous way to say ‘OK, whatever’ and move on with our lives.”

The word also isn’t exclusively lobbed at older people. Young people often use it against one another if they feel another person their age is being closed-minded or says something that sounds like it came from an older generation.

“Boomer can be applied more to personality than really what date you were born,” said Nick Carver, 17.

Luca Brennan, who came up with the idea for the “OK boomer” picture for his senior panorama photo, agreed.

“A boomer is really more of a type of personality, someone who is intolerant to new ideas and who is ignorant to new ideas,” Brennan said. "Stuff like that."

Although teens said they use it with condescending adults online and to tease one another, they said they also direct it at their parents when they feel they’re being unfair or overly conservative.

“The one person I’ve called a boomer is my father. I think it describes him accurately because he’s very stubborn and old-fashioned,” Brennan said, adding that his father became defensive when the teen replied to criticisms with the phrase.

Brennan said when he was recently critiqued by his father for always being on his phone, he used the phrase “OK boomer,” and then explained that older generations were responsible for things like “climate change, the 2008 financial crisis," and "several wars we should not have been in.”

“Boomers come from a different era. They’re behind the times. They’re out of touch,” Brennan said.

The rise of the phrase “OK boomer” mirrors the growing anger among young people at the older generation’s passivity for the issues facing the world, not only today, but for the issues that young people say will be left to them to figure out once they become adults.

“I think of 'OK boomer' as kind of saying, you’re a hypocrite,” Carver said. “You’re criticizing us for everything we’re doing wrong when look at what you created of our world.”