The idiom is "the brightest and the largest".

The moon on August 15 is the biggest, roundest and brightest time of the year, hence the idiom.

Pronunciation zhèng dà guāng míng?

Interpretation behavior is decent, open-minded.

Origin Song - Zhu Xi, "Answer to the Book of Lv Bogong": "For the most part, the hearts of the sages are upright and bright, and are open to the four winds."

Near synonyms

Bright and upright? [ guāng míng zhèng dà ] describes open-mindedness and decent behavior. It is also said that it is bright.

Origin: Song - Zhu Xi, "Zhu Zi Yu Zi - Yi Jiu": "The words spoken by the sages; they are bright and upright; they must first pay attention to the bright and upright program entries."

Antonyms

Sneaky? [ tōu tōu mō mō ] state word. Describes doing something in secret, not daring to let people know about it.

Origin: Qing Cao Xueqin, A Dream of Red Mansions, 80th episode: "What do you want to do and I say, don't sneak around, not useful."