The Wikipedia page for "Yo mama" joke details the joke's construction, ancient times usage, function, and modern use. These are one-liner humor forms that insult someone's mother, often leveraging sentiments of parental respect and frequently suggesting promiscuity or obesity. The page also highlights how these insults can sometimes provoke violence and that slang variants exist based on the speaker.
The Wikipedia page for “yo mama” jokes has a savage “ancient times” section.