Fromper
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The main ones I can think of that are specific to these boards are the abbreviations for the most commonly used Pathfinder books: CRB is Core Rulebook, APG is Advanced Players Guide, UM is Ultimate Magic, and UC is Ultimate Combat. Also, Pathfinder is sometimes abbreviated to PF and Pathfinder Society is PFS. When talking about D&D, different editions are generally referred to by number: 1e, 2e, 4e for 1st, 2nd, and 4th editions, while 3e is usually specified as 3.0 or 3.5, depending on which one you're talking about.
Otherwise, hunt the interwebs for lists of common web forum abbreviations, as there are plenty that are widely used all over the web.