Bagster
It stands for “fishy”, smart guy.
And, by the way, I learned about the acronym “BFD” somewhere on Free Republic, along with quite a few other notable expressions over the years.
Bagster
OK...so I’m assuming you aren’t as old as I am. As a teenager (1963) “BFD” sweatshirts were a “thing” even if we had to make them ourselves with spray paint. Banned at school, of course. Still have one of those tucked away in the attic - along with my “Rat Fink” sweatshirt.