Jargon for porn.
Usenet, IRC Pornography. Originally this referred only to Internet porn but since then it has expanded to refer to just about any kind. The term comes from the warez kiddies tendency to replace letters with numbers. At some point on IRC someone mistyped, swapping the middle two characters, and the name stuck. It then propagated over into mainstream hacker usage. New versions of the Mozilla web browser internally refer to the image library as libpr0n?. Compare filk, grilf, hing and newsfroup.