A (fuckin’ A)
It means 'absolutely' or 'damn straight.' It's like when Aussies say 'Bloody Oath.' Don't overthink it. It's just a word. Don't make it complicated.
'You're gonna pass the test.' 'Fuckin' A.'
'I'm gonna eat all the pizza.' 'Fuckin' A.'
'I'm gonna win the game.' 'Fuckin' A.'