It confuses me each time I see it, because on other non-weight-related boards, OP is "original poster" or "original post", and you use it during a long discussion that rambles here and there to point your argument back to the post that started the thread.
It's not a stupid question. There is a thread in another area of the forum that has a whole list of what some of the abbreviations mean. I don't know how to make a link to there, but if you go to the Low Carbohydrate Lifestyle section, then go under where it says ~ Low carbohydrate frequently asked questions ~ then under accronyms ~ you will find the list.