If you know, speak up. If you don't know, lean against the pillar and listen.

First of all, it must be affirmed that this is a Vietnamese proverb, but some people misunderstand the meaning of this sentence.

At the latest from the beginning of the 20th century, in Vietnamese, there appeared a proverb written in Nom script: "別辰疎啐 庒別辰 預榾麻𦖑" (If you know, then speak out, if you don't know, then lean against a pillar and listen). This sentence was recorded in the book Nam quoc phuong ngon tuc ngu bi luc , printed according to the woodblocks of the Duy Tan Giap Dan year (1914) by Quan Van Duong Tang Ban. This may be the original sentence, later the word "chang" was replaced by the word "khong" to become the sentence: "If you know, then speak out, if you don't know, then lean against a pillar and listen", which was recorded in the book Túc-ngữ phong-dao by Nguyen Van Ngoc published in 1928 (sentence 226, p.36).

From then on, this sentence was still quoted in the same way, meaning that the word "not" had completely disappeared as in the Vietnamese dictionary of the Khai Tri Tien Duc Association, Trung Bac Tan Van (1931, p.575).

"If you know something, speak up. If you don't know something, lean against a pillar and listen." This means that when you know something, speak up. If you don't know something, sit still and let someone who knows explain and talk about it.

In the Nom script, "thao" (𠽔) means "to speak" or "to explain, to report", to express to a superior or someone you respect, for example: "Sit down so I can bow to you and then I will tell you" (Truyen Kieu); "Say that my parents warned me. As a girl, don't chew betel leaves from others" (Ly Hang folk song) .

Thot (啐) also means "to say": "The flower smiles, the jade says gracefully"; "We have sworn together many times" (The Tale of Kieu) . In general, "thao thot" is a compound word, meaning "to speak, to send".

However, some people have confused the meaning of this sentence because they have misread and misspelled the word "thưa thot" as "thưa đá". Please note, "thưa thot" has a completely different meaning from "thưa đá" (Nôm script: 踈率 or 䜹闼) - a compound word meaning "sparse, scattered". For example: "The thatched cottage is sparse, the winter dew is cold and cold" (Thien Nam ngu luc ngoai ky) or "thưa không, lonely": "The moon swears to be still there, bare. Dare to be far away but the heart is sparse" (Truyen Kieu).

Now, comparing with Chinese, we see that there is an idiom "tri vo bat ngon" (知无不言) which has a similar meaning to the first part of the sentence "Biet thi thi thi...". This idiom means "Know then say", quoted from Thoi Quang story in Bac su , a book written by Ly Dai Su about the history of the Northern dynasties and many other historical documents, from the Northern Wei Dynasty (AD 386) to the Sui Dynasty (AD 619).

Later, the saying "Knowing without speaking, speaking without speaking" (知无不言, 言无不尽) appeared, meaning "Knowing without speaking, speaking without speaking will tell everything". This saying originated from the chapter Yuan Lu in the book Hanh Lun by Su Xun of the Northern Song Dynasty. We would like to quote the original text for your reference: "Knowing without speaking, speaking without speaking will tell everything. A hundred people praising us will not make us closer, a hundred people criticizing us will not make us more isolated".

Source: https://thanhnien.vn/biet-thi-thua-thot-khong-biet-thi-dua-cot-ma-nghe-185250530223413437.htm