It is a pun on the word 婦女子 (fujoshi: high-class girls) by replacing the first word to read 腐女子 (fujoshi: literally, "rotten girls").
It is a term used by girls to self-mock themselves about being "rotten." That meaning - fujoshis are the ones that always think of BL (boy's-love) relationships, yaoi-like ideas, have extreme curiosity in portraying two sexy guys together (like Cloud x Sephiroth), etc. etc. Basically, it is the terminology used to describe "yaoi-loving fangirls" as they are called outside of Japan. In many ways, Haga Reiko and her friends in "Comic Party" and Ohno Kanako from "Genshiken" can be portrayed as a stereotypical fujoshi.
Hoping that this term catches on, I'm going to be checking how many American girls are holding placards proclaiming "I AM A FUJOSHI!" rather than "I love yaoi!!" at this year's Anime Expo. wwwww