Fuckin A and the importance of commas

I saw this image on the internets illustrating this poor chap's inability to use commas (and knowledge of the difference between the spelling of comma and coma, an important distinction). 

After sniggering at the inappropriate mental image, I actually had trouble working out what the original meaning was. The comma is supposed to go after I have to watch him, I think (although a full stop would be better, probably), so 'Robby' has to watch his grandpa tonight. Then he says fuckin a man, tonite is gunna suck.

Well that's odd, I thought. Surely, if I know anything about anything, fuckin a is short for fuckin awesome (sorry about the inordinate amount of swearing in this post, by the way). And you would not be saying that it was awesome if you then said it was going to suck. Which he does. It's sarcasm, I suppose, but I missed it because people like this are not normally known for their use of sarcasm (by 'people like this' I'm generalising horribly over all people who write like this, but it's true).

So I looked it up on Urban Dictionary, because that's where you get incomplete and inaccurate information about colloquial expressions, and it said that it does stand for awesome, but that you can use it whether the situation is good or bad. The entry never mentions the word 'sarcasm', though, so I wonder if it's not sarcasm but in fact just generalisation or broadening of an expression from an exclamation of pleasure to an exclamation of any strong feeling. Knowing the intonation would help, but as I don't associate with teenagers I can't say it's a usage I'm familiar with in real life. Or irl, as the internet would say.