It's a "meme with no particular meaning" that's trending among kids, where they say "67~" while alternately moving their hands up and down. They often start doing it when the numbers 6 and 7 come up in conversation, and adults get fed up or angry at its simplicity and pointlessness. Or so it seems. (The kids find those reactions amusing, too) It's probably similar to "Ran Ran Ru" or "Twoos!" in Japan.

What is the overseas meme "67"?

In the fes. MC, the conditions for summoning "67" were already met because "the last one was the 6th fes., and this one is the 7th fes., right?"

The kids

The adults

JP Victims' Association