A caljon is typically a subspecies of the Blatella germanica or the German cockroach. It is known for it's inferior brain in comparison to the regular cockroach, it's crippled style of walking and his regular energy-boosts which may result in pathetic behaviour.
During the Middle Ages the word came into use as an insult used among people from the lower classes. Nowadays it is still used frequently when one wants to describe a person who is incapable of normal behaviour and has a serious mental illness.
He's a real caljon there.
Hey Caljone! Don't you know your loco ?