when something is absolutely insane, or just down right gnar.

meaning someone who is silly or stupid, usually for attention and in a random or sporadic way, involving nonsensical references and actions. usually violent or quick action involved, resulting in general amusement and hilarity of onlookers.

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.

mentally-challenged; retarded; needs psychologic help

Usually one with a mental disorder.
Including anxiety disorders, personality disorders,eating disorders, and much more.
Usuaully used as "crazy"

Derives from the adjective temperamental: a person used to sudden changes in mood.
Because Americans can't spell, the word has been shortened to the last two syllables, but everybody has forgotten about the original spelling. That's why shortening words is always a bad idea.

stupid, dumb, idiotic (from mentally retarded, derogatory)