Yes, we know bleeping out the vowels of curse words is silly, particularly on a site described as "obsessed with funny shit"
but it's company policy for the titles, and we're spine-free bootlicks at heart.

Anyway, ardent truth-mongers The Straight Dope did their best to figure out the phrase "fuckin' A"
The verdict? Uncertain, and a little surly. Witness :
"'Fuckin' A' is short for you bet your fucking ass. You want cites? P*ss on your cites. Sometimes at the Straight Dope we just know."

The origin of "fuckin' A" [The Straight Dope] via z-filter

Also, a guy with a nipple on his leg. No, he isn't being hugged by Tom Cruise, we mean he has a deformity.

Also check out what Googling "fuckn' a" turns up.
NSFW, unless you're working in Amsterdam. They can deal with anything. Amazing people, really.