I always thought the A stood for Awesome, as in Fuckin’ Awesome.
Hate to sound prudelike, but isn’t that sort of language discouraged (nay, strongly discouraged) outside The Pit?
Doubtful, since “Awesome!” is an exclamation of recent mint, while the earliest “Fuckin’ A!” cite we’ve found is from the 40’s.
(To clarify: Of course the word “awesome” is ancient, but its use as a one-word exclamation is recent. Derived from surferspeak, if I’m not mistaken.)
Fuck’n A = Fuck’n Affirmative.
It is Affirmative. Another way of saying “damn straight” or the like. Fuck’n Affirmative.
It comes from military circles and found broad usage after WW2.
Yes!! Oh my god, vindication. Whenever I saw the word “Fuckin’ A” i always would sort of wonder what it meant and eventually I thought it stood for affirmative. Which I was going to post, but then BlueMoonofKentucy posted it first. So I’ll just have to back him up. It’s always just made the most sense to me.
Hmm… I always thought it was “fuckin’ ay,” indicating emphatic agreement. This spelling really leaves no question of the meaning. An alternate spelling would be “fuckin’ aye,” which means the same thing and sounds very much like “fuckin’ a” when spoken with certain accents.
As a statement of my authority in these matters and to give credence to my assertions, I’d like to point out that I am wearing fuzzy socks.
Fuzzy socks!!?? Well, that settles it folks! I defer to the poster with the fuzzy socks.
–Nott, whose cat has fuzzy socks.
Can’t there be a possibility it evolved into something different, including different uses in different regions?
When I was growing up, it was always used in the context of “fucking asshole.” It was used in this context:
highschooler: I just spilled oil on my shirt, fuckin A dude!
I’ve never heard it in the context of agreeing or affirming something.
Just to complicate matters, people sometimes use the expression “fuckin’ A” to express confusion or uncertainty, much the same as they might say “what the hell?” of “huh?”
In such circumstances, “A” seems to be a phonetic rendering of “eh?”
Seems the Straight Dope answered this one recently here. This thread is mentioned specifically.
I’d figured it had to mean “Right on” or something similar ever since I saw The Deerhunter, when Axel used it constantly and always in a positive way.
Is that how it is spelt, or is it “fuckin’ eh”, which is what I have allways assumed it was.
[kiwi accent]Sumilar to the way a Kiwi funishes a suntince eh bro’?[/kiwi accent]*
Thus turning it into a rhetorical question.
*please dont take offence Kiwis. 
WAG = wild ass guess? Just guessing…
We’ll try not to. We really will.
And yes: WAG = “wild ass guess”.
I have always used “Fuckin A” as another way of saying"Right On" 
ummm actually im very experienced on this phrase being as that i use it everyday and it is not used in agreement it is actually used from fustration and i say freaking a instead of fucking a but same thing anyway imma use ur example
“You smell funny”
“freakin a”
YOU SEE WAHT I MEAN!!??
Poster A: Worst zombie reanimation ever
Joey P Agreeing with Poster A: Fuckin’ A!
I had a friend that would respond to “Fucking A” with “Fuck a B, it has two holes” and then encourage the other person to continue through the alphabet with her to see how far they could make it without stopping.
I missed that post, as soon as I saw how old it was I just scrolled down to the bottom to see what brought it back to life.
IME it can be used as either an ejaculation of happiness or of disappointment.
“Fuckin’ A! She said she’d sleep with me!” (I am very pleased that she has done so)
“Fuckin A! He came already!” (I am very displeased that he has done so.)