This phrase has commonly been misinterpreted as the complete opposite of its actual meaning. when someone asks "Are you mental?!" it means that they are asking if you ARE in fact mental and your brain is functioning PROPERLY. In todays society it has been disregarded as a phrase that implies being "mental" is a bad thing, when in fact being "mental" means your brain is working correctly.
John Madden: Are you MENTAL?!
Generic Football Player: Yes actually, I am!