'fucking A' is considered vulgar English. It can function as an interjection (!) or as an adjective. It is used to express objection as well as to describe something or someone as top grade A material.
As far as I know, there are two ways to use it:
1. A stands for alright
Definition: Used to express objection.
Ex: Fucking A (right) you are going to do what I say!
2. A stands for alright, amazing, or A as in Grade 'A', top quality
Definition: Used synonymously with amazing, wonderful, great.
Ex: That Alien movie was fucking A (alright)!