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1.
A question asked in disbelief with an extremely sarcastic tone questioning someones actions or statement. It is a way to point out someones stupidity in the form of a question.

total disbelief or dissapointment in how someone did or said something.

Note: it is a very hard inflection that goes up to indicate the question
Hall: i definitely think that girl is hotter than jessica simpson.

Bush: that girl in our science class she's barely average.

Hall: I think she's amazingly gorgeous and makes simpson look ugly?

Bush: Are you serious?
by Randoni April 05, 2009
 
2.
The verbal tic of dunderheads, morons and wannabe Californians (read: Seattle denizens and their ilk.) It is not intended as an actual question but rather is the reactionary knee-jerk of someone who does not think before opening the mouth.
Me: I just got this latte for uber cheap at the Starbucks on the corner.

Idiot I'm talking to: Are you serious?

Me: No, I'm just pulling your motherfucking leg. Of COURSE I'm serious!
by grammarcopkyle August 28, 2010
 
3.
An expression uttered subsequent to an unforeseen response; used in a joking sense; can carry a sarcastic tone; usually accompanied by a dramatic step back from the person; although it carries a question mark it doesn't require an answer.
Paul: "How'd ye get on in the match today?"
Shane: "We lost by 4 goals..."
Paul: "Are you serious!?"
by John O' Driscoll June 05, 2007
 
4.
similar to 'r u sure'
jeff: lisa goosed warren last night
Hardy: are you serious
by Big John Stud April 11, 2005