Distinguished from the phrase, Fuckin' A, fuckina is one word with the short vowel sound on the a (uh)...usually dragged out a little.....Fuckinuh
I literally here this every day...Almost everyone I know says fuckina....
It is used as an adjective, as when someone says, "Let me have that fuckin' 1/2 inch socket" only they need a second to determine exactly what they want to say...."Let me have that fuckina 1/2 inch socket."
Another example, "I gotta get one of those new fuckina Samsung Galaxy phones."
This may be a better spelling...Adjective, used to get a split second to say exactly what you mean...There is sometimes a slight pause right after saying it...
"I'd like to get one of those fuckinuh Samsung Galaxy cell phones, they're awesome."