Where does "fuckin' A" come from

I'd heard it in Office Space, and situationally in real life. I've used it, and it flows off the tongue rather well. But it doesn't really make any sense...

-The phrase is first recorded in print in 1947, in Norman Mailer's World War II novel The Naked and the Dead. Mailer has a character say, "'You're fuggin ay,' Gallegher snorted."  The actress Tallulah Bankhead claimed she met Mailer at a party and said, "So, you're the guy who doesn't know how to spell fuck." (The story is sometimes told with Dorothy Parker as the speaker.) Mailer told an interviewer he never met Tallulah Bankhead, and in any case he knew how to spell four-letter words--the euphemism was used in order not to offend the sensibilities of readers in 1947. "Fuggin ay" suggests how hard it is to come by cites of one of the (formerly?) most offensive words in the English language--the next print citation of the phrase is from 1955, ". . . freaking-A loud sneeze," where it meant "goddamned." 

The phrase has been an adverb, an adjective, an infix (an affix stuck into the middle of a root, e.g., "abso-fuckin'-A-lutely"), and an interjection in its lifetime. It has been used to mean "yes, indeed," "absolutely (correct)," "splendid," "very well," and "fucking" or "goddamned."

Having established that the term existed in WWII (and no doubt before), we now want to divine what the "A" means...

via The F Word by Jesse Sheidlower