What is the meaning of "Ziqi Donglai"?

Zǐ qì dōng lái

Meaning--Legend has it that before Lao Zi crossed the Hangu Pass, Yin Xi, the commander of the Pass, saw a purple gas coming from the east and knew that there would be a saint crossing the pass. Sure enough, Laozi came riding a green ox. In the old days, it was a metaphor for auspicious signs.

Source - "The Records of the Grand Historian (史记-老子韩非列传)".

Expanded Information

Allusion--

Lao Zi was very learned and held an official position in charge of books and canons in the Zhou Dynasty. When he was in his seventies, the world was in great turmoil, and there was a great struggle between the lords

The "Purple Energy Coming from the East" was one of the great triumphs of "Laozi's Exit from the Passage", which has been passed down through the ages and recited from generation to generation.

Chinese idioms--

Purple gas is purple gas, and the ancients believed that the color purple was associated with being an official. Many official clothes are purple. Purple qi is mainly used for plaques on the main entrance of a scholarly family's home.

References:

Ziqi Donglai - Baidu Encyclopedia