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/
That might just be California. In standard “10” police codes, it’s 67. https://www.police1.com/resources/articles/police-codes-VqFqvwMyjl6GES0f/