hmmm...I have mused and posted on "fuckin' ***" before... I first heard it in about 1969 as 'aye'...meaning "absolutely yes" ...and a web search will get you many of that usage, as well as plenty of the other. I am guessing that 'aye' is the origin, as aye=yes, but that someone saw it in print, did not know how to pronounce 'aye'...said 'ay' or, as it usually shows up in print "Fuckin A!"...and off we went!

Stuff often changes like that...as in Mondegreens.

I have seen no definitive study, but I can't imagine how 'ay' or 'A' would appear first. Any other ideas would be interesting.