Title text:
In 1899, people were walking around shouting '23' at each other and laughing, and confused reporters were writing articles trying to figure out what it meant.
Transcript:
Transcript will show once it’s been added to explainxkcd.com
Source: https://xkcd.com/3184/
It's been around since the 1980s. If you didn't know it it's not because you're a millennial, it's because you weren't part of the right subcultures when you were young / teen / 20s.