The term is a metaphor about giving the finishing touch, which means providing critical details or key words in an artistic or literary work in order to lend it charm and aesthetic conception. Mencius (372?-289 BC) believed that when observing a person, one should look directly into his eyes because the eyes reveal his nature, be it good or evil. When painting portraits, Gu Kaizhi (345?-409) in the Eastern Jin Dynasty did not add pupils to the eyes in haste. He stressed that the key to painting a vivid portrait lied in painting the eyes. Zhang Sengyao, a painter of the Southern Dynasties, was well known for his excellent painting skills. Legend has it that his painted dragons flew into the sky as soon as he finished their pupils. The term is thus used by later generations to underline the importance of applying critical touches to add life and charm to a literary or artistic work.

Zhang Sengyao painted four white Chinese dragons on the wall of the Anle Temple in Jinling. But he did not paint pupils to their eyes, saying that once he did, the dragons would fly into the sky. People considered his words absurd and repeatedly urged him to add pupils to the dragons’ eyes. He eventually did it on two of the four dragons. Suddenly, lightning and thunders struck, and the two dragons with pupils added to their eyes flew into the clouds. The other two remained on the wall.

(Zhang Yanyuan: Famous Paintings Through History)