The word is used in online slang, to refer to adult entertainment – be it internet or general – in a secretive manner.
It is also used as a search word for avoiding filters, looking for the actual word. Pron can often be found in filenames, where it is a code not to be understood by everyone.
It is often seen with the letter 0 in place of the “o” in order to make it more undecipherable, as well as written backwards (norp) and written phonetically (prawn).
Pron originates from chatrooms, where it has been probably first seen as a typo, mixing the order of the middle letters.
It gained big recognition and spread into the slang used by the leet (1337) community. This is possibly the point, where the “0” replaced the “o” as letters are often replaced by numbers in this online community.