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 itself.

Similar words for divisions of pr0n that are still in use include:
R@ygold - (pedophylia)
qwerty - (bestiality)
showme - (voyuer)
rorikon/lolikon - (anime child)
momashi - (incest)
BBW - (Big Breasted Women)
etc...

1337 (leet) for pornography. usually the o is replaced by the digit 0, ie: pr0n.

l337 for porn.

n : film or literature designed to stimulate sexual desire syn: pornography, porno

n.Pornography.

adj.Pornographic.

"pron" or "pr0n" is

An accidental or deliberate misspelling of the word 'porn'. Due to the common misspelling of the word, it has been accepted and encouraged amongst the online gaming community.
It was also used to get around keyword filtering such as in AOL's parental control.
This allowed all the children of the world to have access to all the porn (or pr0n) that is nessessary for a healthy childhood.

A l33tspeak codeword for Porn, originally used to avoid keyword filters that might expose hidden FTPs or websites.

1. A misspelling of the word "porn" (short for pornography). Often seen in search engine histories.

2. A popular euphamism for porn, often seen used by wannabe hackers and script kiddies who have to use "1337speak" in order to seem cool. Became popular after the search-engine typo became more common.

hax0r speak for porn, pr0n.