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/
Hmm, it's hard to think of good resources for this. It's just kinda part of community. You could google asl slang and look around in YouTube or something. Or learn asl (or your country's sign language) and join the community.
Found a link with a short quip about 258 lol
https://258hub.ca/about/