Commonly used on TSR (the student room) when you want to rep (give reputation points) to someone but the server rejects your request as you are not allowed to rep the same person twice in too short a period. You therfore write PRSOM to let the person know that you want to rep them but are unable to do so (the server says: please rate some other member before giving reputation to this member).
Means AWESOME, but made for cool 1337 gamers and nerds. Usually reserved for most extreme awesome things. Coined originally by Neil Nguyen during a game of paintball.