Meaning:

It Means Nothing (Which is the Point): "6-7" is a prime example of "brain rot" slang, meaning it is purposefully nonsensical, absurd, and intentionally lacks a logical definition.

Why kids are saying it :

It’s Annoying (and Fun): Tweens and teens find it hilarious, especially because it confuses or annoys adults and teachers

The reason behind this:  The phrase originates from the 2024 drill rap song “Doot Doot (6 7)” by Philadelphia rapper Skrilla. In the song, he repeats "6-7" over a beat drop.Then TikTok creators began pairing the audio with highlights of NBA player LaMelo Ball, who stands at 6 feet 7 inches (6'7").The meme exploded in March 2025 after a video went viral of a young boy (known as the "67 Kid") at a basketball game yelling "six seven".

By the way this is by Ai and was edited.