It's one of those solecisms, like "for free", that are now in constant use in the UK but which make me grind my teeth in annoyance and frustration. I don't consider "fun" to be an adjective, nor "free" to be a noun, but such misuses of the language are imposed upon me umpteen times daily by people who should know better.
"My bad" = "Sorry, my mistake." Except sometimes I don't think they're really sorry, but they have admitted an error. I haven't heard this one as much as a few years ago--maybe it's on its way out (or just around to you!).
I think that the "my bad" phrase has been given a huge boost by Hugh Laurie's character in "House"! I wouldn't ever say it, but it sounds quite comical when he does. TV has a lot to answer for.
"Fun" used adjectivally is something I battle on a daily basis! Perhaps it's time I admitted defeat (as I have on so many other issues)!
"Fun" as an adjective, and "free" and "bad" as nouns, aren't things I'll ever agree to as being correct English, and I'll never use them. Possibly I've reached the stage where I have to accept that the ignorant masses will use them anyway, so I'll just have to grit my teeth, swear a bit, and let them pass over my head.
Introverts unite! (Separately, in your own homes.)