夜yè郎láng自zì大dà

Literal: Yelang thinks itself great
Figurative: To swagger with self-importance out of sheer ignorance of the wider world; unlike 井底之蛙, the stress falls on arrogance, not just on narrow horizons.

Where does 夜郎自大 come from?

It comes from Sima Qian's Records of the Grand Historian, in the chapter on the peoples of the southwest. Under Emperor Wu of Han, envoys searching for a route to India reached the kingdom of Dian, whose king asked them which was greater, Han or his own realm. The lord of Yelang, the largest of the southwestern kingdoms, asked them the same question. According to Sima Qian, since the roads were closed, each thought himself master of a whole province and had no idea how vast Han was. The first to ask was the king of Dian, but the idiom stuck to Yelang. Today it is said of anyone who swaggers, unaware of how small their world is.

Source (出处): 《史记·西南夷列传》

How is 夜郎自大 used? Two examples

他tā才cái赢yíng了le两liǎng场chǎng小xiǎo比bǐ赛sài就jiù觉jué得de自zì己jǐ天tiān下xià无wú敌dí，未wèi免miǎn太tài夜yè郎láng自zì大dà了le。
He's won just two small tournaments and already thinks he's unbeatable; he's getting far too big for his boots.
我wǒ们men在zài本běn地dì做zuò得de还hái行xíng，但dàn跟gēn行háng业yè巨jù头tóu比bǐ，千qiān万wàn别bié夜yè郎láng自zì大dà。
We're doing all right locally, but next to the industry giants we'd better not get full of ourselves.

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