What does an empty letter mean?

the phonetic notation ㄎㄨㄥ ㄏㄢˊ

the basic definition of pinyin k not ng há n

⒈? Refers to a letter without letterhead.

⒉ ? Refers to a short note with vague content. Revised Mandarin Dictionary

Empty letter "ㄎㄨㄥㄏㄢˊ"

1? Letters without words.

⒉ ? A letter asking for articles without postage stamps. Citation explanation

1? Refers to a letter without letterhead.

Quote "Biography of Yin Hao in the Book of Jin": "In the later period, Wen will take Hao as the official book order, and he will tell it in a suicide note, and he will gladly make a promise. I will answer the book, fearing that there are mistakes. There are dozens of people who open and close it, but it is empty, and it is too warm, so it is absolutely rejected. "

⒉ ? Refers to a short note with vague content.

Quote "Southern History, Liang Ji Shang, Emperor Wu Shang": "Shanyang is confused by many people, and it is self-evident to act in advance and retreat, which means that a state is determined by two empty letters."