You're not alone. I've been here quite awhile, and I still run into ones that I don't know. One that was confusing to me for awhile was HG, which I finally deduced to mean Holy Grail.
Yesterday I saw PdN, and had no idea. I gather it means Parfum De New York (a brand) or something similar.
Most of the other ones I see are shorthand for fancy frags or houses. I believe there are posts here and there that have lists of house abbreviations.
I enjoy languages, and I was thinking of putting together a glossary for that. For instance, I take it for granted that eau, aqua, acqua, and agua all mean water, but I can't assume that everybody knows that.