Courtesy of Reuters, which has other ancient jokes as well:
image via flickr user rosemanios
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It is a saying of the Sumerians, who lived in what is now southern Iraq and goes: "Something which has never occurred since time immemorial; a young woman did not fart in her husband's lap."
image via flickr user rosemanios
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Their 10th "oldest" joke is from the 5th century! Pathetic. Terrible, terrible science reporting from Reuters and the media.
http://www.birminghampost.net/news/2008/07/31/wolverhampton-researchers-discover-world-s-oldest-joke-65233-21445845/
I certainly don't know what it means; and I bet if you got 5 anthropologists together, they couldn't agree either.
Or maybe I'm wrong, but the news article didn't illuminate anything.