  The Conceited King of Yelang 夜郎自大

  In the Han Dynasty, there was a tiny country called Yelang on the southwestern border. Small though it was, its ruler was quite proud of his country, thinking it big and powerful. Once a Han envoy visited Yelang. The ruler asked him: 'which is bigger, Han or Yelang?' 
  在汉朝时，西南边境上有一个小国，叫夜郎国。这个国家虽然很小，但是国王却很骄傲，自以为他的国家很大，很了不起。有一次，汉朝的使者访问夜郎国，国王问使者："汉朝与我们夜郎国比较，哪一个大呢？" 

  Later this idiom came to be used to refer to those who are capable of nothing yet are conceited.
  后来人们就把本来没有什么能耐，却自以为很了不起而瞧不起别人，说成"夜郎自大"。