I am not a native English speaker so I wouldn’t want to make a guess. Which is correct? Or are they both correct?
And example would be:
What a stupid ass question. vs What a stupid assed question
And should it be hypenated?
Well, it’s slang, so there probably is no absolute correct form.
The correct form would probably be stupid-assed question, hyphenated so you know the two adjectives are paired together.
(I am not a grammarian!)
Again, it’s a slang term and not proper English, so the rules are hard to define.
They should definitelty be hyphenated; they’re compound adjectives.
There’s a subtle difference, but it’s very hard to describe.
The -ed form is common when adding adjectives to things: three-eyed fish, one-legged man, and so on. By hiding the distinctive adjective–the number of legs–slightly, it puts more emphasis on the man. On the other hand, if you say “one-leg man”, there’s slightly less emphasis on the man, and more on the number of legs.
Urban dictionary is helpful with its def of dumbass
It is the dumbass who asks dumbassed questions.
Same with stupidass/stupidassed.
If I had to write this down I would use stupid-assed, big-assed, crazy-assed, etc. I suppose you could consider usage like stupid-ass to be dialect 
But you need the hyphen.
Hmmm. After almost an entire minute of thoughtful deliberation, I’d say “stupid-assed” sounds like you’re calling somebody’s ass stupid, whereas “stupid-ass” is more along the lines of assigning a particular type of stupid, i.e. shorthand for “as stupid as an ass”.
I’ll [del]drink to[/del] agree with that. And the hyphen is good, because “stupid ass question” looks like an ass question that’s stupid (“Do these pants make my ass look big?”)
The “ass” qualifier is idiomatic slang, as in “grown-ass man” or “big-ass ham.”
The way you would type it out is “stupid-assed”. The way you’d say it is “stupidass”, said fast and kind of run together with the object you’re modifying–like “stupidass raincoat!”.
It’s slang though, so there are no hard rules. But this is a good rule of thumb for a non-native speaker.
A stupid ass idea is the type of idea a donkey would come up with.
A stupid assed idea is an idea whose ass (derriere) is stupid.
I would both say and write it as stupid-ass. Sunspace’s description of the effect of adding the -ed is a good one.
And now that I think about it, I use that term a lot. Indeed it almost replaces the word stupid for me.
Well, according to Scott Walker’s Jacky (or at least according to the transcripts at several lyric sites), it’s “stupid ass”:
If I could be for only an hour
If I could be for an hour every day
If I could be for just one little hour
Cute in a stupid ass way
I am a bit suspicious of the transcripts though, because Scott very clearly does not sing “Cute” just once, but twice: “Cute cute”. I think all these lyric sites just copy off one another. Still, it does sound as if he sings “stupid ass” (not “assed”) to me.
(Also, apparently, these English lyrics were not written by Scott, but by one Mort Shuman. Its really brel, though.)
Note also, that in Britain, an ass (pronounced with a short a) is a donkey (literally) or, metaphorically, a foolish and/or stubborn person, whereas an arse (with a long a) is literally a backside, bottom, butt, or bum, and metaphorically an unpleasant, pettily malicious person. Ass (in Britain) is a considerably milder and more affectionate insult than arse.
In America, only the ass spelling seems to be used, and although both pronunciations seem to occur, I don’t think variations in pronunciation affect the meaning (they may be regional). I rather think the “donkey” meaning is now effectively obsolete in America, but the meanings “backside,” “fool” and “nasty person” are all still there.
To complicate matters, Scott Walker was an American but had most of his career success in Britain, and although the lyric transcript sites all say “stupid ass,” be pronounces it much closer to “stupid arse.” The song was banned by the BBC when it was first out, in 1967, and I am pretty sure that the word “ass” would not have been seen as a reason for censoring a song in those days; but “arse” just might have been (although IIRC, the main reason given for the censorship was the mention of “authentic queers and phony virgins” elsewhere in the lyric).