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!