Rot13 Joke Encryption
Since the start of the net, people have complained about potentially offensive postings.
Many years ago (the early 80s) somebody came up with the idea of trivially encrypting jokes. The idea was that if a joke was encrypted, and looked like garbage to the ordinary eye, everybody would have to run a special program to decrypt the joke. If you did this, it was argued, you could hardly complain about being offended. You were warned that the joke was bad and you still went and decrypted it?
Over time, the standard became known as rot13. It's one of the simplest children's cyphers. You replace every letter with the letter 13 later in the alphabet. A is replaced with N, B with O, all the way to replacing Z with M.
As this became popular, most newsreaders added a command to decrypt jokes that were encoded in rot13. That sort of defeated the point of people having to go out of their way to read a joke, but the essential message was still there -- responsibility for one's own actions, and not complaining about being offended if you had to work at it.
Rec.humor.funny has tended to use rot13 not for sex or dirty words but for political correctness -- the only thing likely to cause real complaint on the net.