67 came from a song lyric, then a kid made a tiktok where he frantically said "six seveeen!! six seveeen!!" to the camera while doing a gesture with his hands. and because kids love pattern recognition, when they see the number it's easy to point it out and do the gesture. and so the meme spread.
the humor comes from it being nonsensical; the joke is on anyone who's either tired of it, or never found it funny at all, so i don't really see it going away anytime soon.