Originally a Buddhist word, it refers to a quiet and subtle realm. For example, "I have to think about the sky ... this day's centering is extremely quiet and wonderful, without any rough thinking, so it is called no thinking." "Even if you get all the quiet and wonderful joy from the pain of birth, illness and death, the fruit of poverty is also virtuous." See Ding's Dictionary of Buddhism.
The realm pursued by China's traditional calligraphy has something in common with this, so some calligraphers often write these two words.