many think it stands for fuck all. really it originally ment 'sweet fanny adams'. fanny adams was a young girl murdered by a solicitor's clerk named Frederick Baker in the town of Alton, Hampshire, England. The expression 'Sweet Fanny Adams' refers to her and has come, through British naval slang, to mean 'sweet fuck all' or 'nothing at all'. before it ment 'fuck all' it was used instead of sayings like 'oh my god'
dave - hi jim what you doing today?
jim - sweet f.a.