Its actually pr0n, and its spelled this way so that it doesn't show up when someone searches the word "porn".
It was introduced as a masking tool to prevent people from logging your involvement in less than reputable activities. Much like the original
intent behind 1337 speak
Similar words for divisions of pr0n that are still in use include:
qwerty - (bestiality
showme - (voyuer)
rorikon/lolikon - (anime child)
momashi - (incest)
BBW - (Big Breasted Women)
Including pr0n in Word_Perfect_help_file_00trpr0n14.pdf makes it hard to realize that the file is actually Tiffany_Shocker.avi renamed; except to the person who masked it of course.